Equalizing Marginal Utility per Dollar Spent

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Equalizing Marginal Utility per Dollar Spent발음듣기

In the last video we thought about how we would allocate our five dollars between chocolate bars and fruit.발음듣기

And the way we did it was very rational: we thought about how much bang we would get for each buck.발음듣기

And we saw starting off with our first dollar we got a lot of bang for our buck.발음듣기

And this is just another way of saying bang for the buck: marginal utility per price.발음듣기

So we got a lot of utility per price starting off with that first chocolate bar, a little less for that next chocolate bar, but still more than we would get for a pound of fruit.발음듣기

Then more for the next chocolate bar.발음듣기

And then, and only then, did we start buying some fruit, some pounds of fruit.발음듣기

What I want to do in this video is generalize it.발음듣기

I want to think about maybe a more continuous case where we can buy very, very small increments of each of the products.발음듣기

It doesn't have to be in chunks, like chocolate bars.발음듣기

And what I'm going to do is plot the marginal utility per price, which is really bang for your buck, on the vertical axis.발음듣기

This right over here, on this axis.발음듣기

This is the marginal utility per price.발음듣기

And let's say it also goes from zero to 100.발음듣기

So that would be 50.발음듣기

And the numbers don't actually matter so much here.발음듣기

And then this will be dollars spent... dollars spent, or your buck.발음듣기

So this is bang for your buck, and this is your buck.발음듣기

This is one, two, three, four, five and six.발음듣기

Now we're going to do two arbitrary products.발음듣기

So let's say one product looks something like this.발음듣기

And once again you have diminishing utility as you get more and more of that product.발음듣기

In the case of fruit, the more pounds of fruit you get the more tired you get of fruit, the less fruit you need or the less you want fruit for that next incremental pound.발음듣기

It could be anything; this is true of most things.발음듣기

So this is Product A.발음듣기

It could be a service as well.발음듣기

So Product A. Let's write it this way.발음듣기

So this is the marginal utility for A per price of A.발음듣기

Now let me get another product right over here.발음듣기

So let's say my other product would look something like this.발음듣기

So this is my marginal utility for Product B per price of B.발음듣기

So it's really saying Bang for the Buck.발음듣기

So just to start off - and I won't even constrain how much money we have.발음듣기

I just want to think about how we would spend that money.발음듣기

So if I were to spend - if I had a penny, where would I spend that penny?발음듣기

And I'm assuming I can buy these in super-small chunks, as small as maybe the penny or even fractions of a penny.발음듣기

So if I just had a penny, and I think about 'where am I getting the best bang for my buck for that penny?'발음듣기

I'm clearly getting it with Product A.발음듣기

So I would spend that penny on Product A and I would get this much bang for my buck.발음듣기

Which would be this entire part, this entire part right over here.발음듣기

Let me color it in.발음듣기

So first I'll spend it right on A.발음듣기

Let me do it in a color that's more likely to be seen, this blue color.발음듣기

So I'll spend it on A. In fact, where would I spend my first dollar?발음듣기

Well, for the whole first dollar I'm getting a better bang for my buck on A.발음듣기

So my first dollar, I'll spend on A. And the total utility I'll get is actually going to be the area under this curve.발음듣기

It's going to be this whole area: it's going to be dollars times marginal utility price.발음듣기

That would obviously give you the area of this rectangle right over here.발음듣기

The reason why it wouldn't be the area of this larger rectangle - it would just be the area under the curve - is you're not getting the 100 marginal utility per price for the entire dollar.발음듣기

It's going down the entire time.발음듣기

And so your actual total marginal utility is just the area under this.발음듣기

And if you take calculus you'll get a better appreciation for that.발음듣기

But let's just think about once again where our dollar is going to be spent.발음듣기

Actually, even after we already spent a dollar our next penny we'd still want to spend on Product A because we're still getting more bang for the buck.발음듣기

We're still getting more bang for the buck all the way till right around there.발음듣기

Now something interesting is happening.발음듣기

So we've spent about two dollars.발음듣기

We spent our first two dollars all on Product A because we're getting more bang for our buck, even though that bang was diminishing every penny or fraction of a penny that we spent.발음듣기

But now, where will we spend our next penny?발음듣기

Well, we could spend it on Product A again.발음듣기

But look.발음듣기

We can get about the same marginal utility spending in on Product B.발음듣기

So we can jump right over there and spend it on Product B.발음듣기

Now where could we spend our next dollar?발음듣기

Well, we get about the same marginal utility whether we spend it on a little bit more of Product B or a little bit more of Product A.발음듣기

So we could do either.발음듣기

If we spend a little bit too much on Product A then we could have gotten more marginal utility spending on Product B.발음듣기

So what we would do is, once we've gotten to this threshold right about here we actually are going to spend every incremental fraction of a penny we're actually going to want a split between Product A and Product B.발음듣기

If we spend too much on one and we go down this curve we could've gotten higher utility on this one.발음듣기

If we spent too much on this one we could get higher utility spending on this one right over here.발음듣기

So there's a very interesting phenomenon here.발음듣기

Assuming that we eventually spent enough that we buy some of both - obviously we started just buying Product A because it had higher utility, at least for those first few dollars.발음듣기

But assuming that we end up buying some mix of the two, which we do end up spending if we spend more than $2, there's an interesting thing: the marginal utility for a project - the marginal utility for B or the marginal utility for price for B that I spent on that last little increment is going to be the same as the marginal utility per price for that last increment of A.발음듣기

So if B was, I don't know, if it was fruit and let's say A was chocolate but we could buy it in very, very small increments we're saying for that last fraction of a pound of fruit you're spending or getting the same marginal utility per price as you're getting for that last fraction of a bar or fraction of a pound of chocolate.발음듣기

So there's a general principle, and it really just comes from this very straightforward thing: that as soon as you can get better marginal utility on the other one you start spending there.발음듣기

But then they start to look equal and you would keep dividing your money between the two.발음듣기

So the general principal: if you're allocating money between two goods for that last increment - not across the board, just that last increment (that's why the word marginal is so important) - for that last ounce of chocolate vs. that very last ounce of fruit the marginal utility per price for that last increment of one good would be the same as the marginal utility per price of the second good.발음듣기

Now I really want to emphasize what this is saying.발음듣기

This is not saying that the marginal utilities per price of the two goods are the same.발음듣기

And not even that one is better than the other.발음듣기

This is just saying as you spend money - let's say you spend enough money to buy both - at some point you're going to get to a threshold where you're neutral between the two, where the marginal utility per price is the same for an incremental of B vs. an incremental of A.발음듣기

And at that point you're just going to keep switching between the two products.발음듣기

Because obviously if you focus too much on this right over here - let's say you focus, at that point you switch and just start buying a bunch of Product B over here.발음듣기

Well, that didn't make sense because you were buying Product B when you could have gotten higher marginal utility buying some of product A.발음듣기

And that's the same reason why you didn't just keep going down A, because you could've gotten higher marginal utility over here.발음듣기

This is closer to, I don't know, 75 while you're only getting 70 right over here.발음듣기

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