Another GRE question?

Question

A 4 cup mix of 1/5 sugra and 4/5 flour has 1cup added. If this cup is all sugar, what is the ratio of sugar to flour expressed as percentages?
a)18%: 64%
b)36% :64%
c)50% : 64%
d)64% : 36%
e) 80%: 20%

Answer 1

b)36% :64%

Before adding the cup of sugar we have

sugar:flour = 4(1/5) : 4(4/5) = 4/5 : 16/5

After adding one cup of sugar we have:

1 + 4/5 : 16/5 = 9/5 : 16/5 = 9 : 16 = 36% : 64%