Which equation does not belong with the other three:

5y=2x

y=2 fifths x

10y=4x

5xy=2

5xy = 2 does not belong, because it doesn't simplify to 5y = 2x like the others. The second one is done when you divide both sides by 5, and you get y = 2/5x, which is true. The third one is done when you multiply both sides by 2, which is 10y = 4x, which is also true. But for the fourth one, to get x to move, you need to divide x from both sides, which is NOT 5xy = 2, it's actually 5y/x = 2. Hope this helps :D