4 is right.

I don't know what on earth you did with #1, but here's some basic hints for it:

5x(75x+32y)=5xy
375x^2+160xy-5xy=0
x(375x+155y)=0

So it's a second-degree equation, and one x has to equal 0, but see if you can figure out what the other one equals.

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram