It's just obvious from the first order conditions,

从一阶条件下
from this marginal utility conditions,

也就是边际效用条件下
they're called first order conditions,

他们被叫做一阶条件
from this equating marginal utilities.

从边际效用
That was the crucial trick.

它无疑是一个很重要的方法
So that's a trick that you have to internalize and from now on

这个方法你需要好好消化
that's all you have to know that C's going to spend 3 quarters of

你需要知道C将花3/4的钱在
his money on X, 1 quarter of his money on Y and

X上 1/4的钱在Y上
D's going to spend 2 thirds of her money on X and 1 third on Y,

D将会花2/3的钱在X上 1/3的钱在Y上
but supply has to equal demand.

但供需要相等
What is C?

C是多少
What is he actually buying?

他买进了多少