Considering 2nd condition as x= y^2..

we need to find out x-y

1. not suff..
if x =10 and y = 10 then x - y = 0
if x = 11 and y = 9 then x - y = 2

2. x=y^2 not suff

if y = 4 then x =16 which satisfies x+y = 20
only 1 combination of y and x satifies both x = y^ 2 and x + y = 20.
so x - y can be determined i.e. Suff.

Answer is C

harsh.champ wrote:If x and y are both positive integers, how much greater is x than y?

1. x + y = 20
2. x = y²

we have to calculate x-y
St.1 ) x + y = 20 =========not suff.
St.2) x = y² =========not suff.
combined) y² + y = 20 --> y² +5y - 4y -20 = 0 ---> (y+5)(y-4) = 0 --> y=-5,4 but x and y are both positive integers
so y= 4 hence x= 16 so x>y ========= suff.
Ans C.