The best way to do this is to make up values for the sides.
Say the lengths of square B's sides are each 4 cm. That means A's sides are 2 cm.
So, the area of square B is 4^2 = 16 cm^2, and A's area is 4 cm^2. We can see the shaded area is half of A, so it's 2 cm^2.
What percent of 16 is 2?
2 / 16 * 100 = 12.5%
Your answer would be 15. You first would move the "pretend" decimal to the left 2 times. so it would end up being 0.25. So then you would set up your problem. 0.25 x 60 =. Which should give you the answer of 15.
If we're trying to find the hypotenuse we'll take the

Getting 35.
35
x − y = 0 = difference
x + y = 4 = sum
Can be solved by the sum and difference rule (most of the time without pen and paper)
x=(sum+difference)/2=0+4/2=2
y=(sum-difference)/2=(4-0)/2=2