the marginal utility of X to the marginal utility of Y

边际效用的比所决定的
because those are the,

因为他们是
you know, when you give up a unit of X you're losing the marginal utility of X.

当放弃一单位x你就失去了x带来的效用
When you're getting a unit of Y you're getting the marginal utility of Y.

当你获得一单位y就同时获得了y的效用
If you can trade them off in the market at 3:1

当你能在市场上以三比一交换x和y
you optimize when, in your own personal evaluation,

当你拥有它们所带来的效用是三比一时
you're trading them off on the margin at 3:1.

你就实现了最优
You really follow that?

真的明白了吗
That's an idea that took fifty years to figure out

这个花了五十年时间才被弄明白
and you claim you figured it out now in five minutes,

你说你们用了五分钟就明白了
so that's good.

很好
So you'll have a chance in the problem set to get practice.

在作业里面你们有机会联系一下