Force of gravity between two objects

[BK715]

Dear Manhattan GMAT instructors

I am struggling with the following problem (Source: The Princeton Review, 1,037 practice questions, 2012; quant diagnostic question 11):

The force of gravity between two objects in space is directly proportional to the product of the masses of the objects and indirectly proportional to the square of the distance between them. I fate distance between two objects is doubled, by what percent must the product of the masses increase if the force of gravity is to remain the same?
(A) 50%
(B) 100%
(C) 200%
(D) 300%
(E) 400%

According to the solution key, the correct solution is (D). I get to (E) though.

What am I missing in my attempt? If
G = Force of gravity
M1 = Mass object 1
M2 = Mass object 2
D = Distance

G = M1 * M2 = 1/d^2
if i now double the distance --> 2d --> my new equation is 1/4d^2 = x * M1 * M2 and then I get to x = 4 in order to keep G constant an get back to my original equation

Thanks
BK

[RonPurewal]

the percent increase from 1 to 4 is only 300%, not 400%.

remember that you have to take the increase as a percentage of the initial value.

400% is the answer to a different question, namely, "the new product of masses must be what percent of the original product...?"

[RonPurewal]

oh, and, by the way, there's no such thing as "indirectly proportional". the correct term is "inversely proportional".

("indirectly proportional" seems to suggest that something is proportional to some second thing, which in turn is proportional to some third thing. but, of course, in that case all three things would just be directly proportional.)