If a consumer allocates her income between two goods, $x$ and $y,$ then she will be happies when

$\frac{?}{MU}\frac{x}{P}x\frac{?}{M}=U\frac{y}{P}y$ and she is below her budget constraint

MUx $=$ MUy and she has spent all of her income

MUx$/$MUy $>P\frac{x}{P}y$ and all of her income is spent

$\frac{?}{MU}\frac{x}{P}x\frac{?}{M}=U\frac{y}{P}y$ and all of her income is spent

$\frac{?}{MU}x=$ MUy and she is below her budget constraint