By "dirty", do you mean a lot of the numbers were decimals? If you don't feel as comfortable working with decimals as you do with integers, one thing you can do is work the problem in terms of cents instead of dollars. That is, think of it as 600 cents instead of 6.00 dollars and 10 cents instead of 0.1 dollars.

C=number of cupcakes bought
x=Price of cupcake in cents
D=number of doughnuts bought
y=Price of doughnut in cents

Cx+Dy=600

Statement 1: 2y=3x-10. INSUFFICIENT.

Statement 2: x+y=70. INSUFFICIENT.

Statements 1&2:

3x-2y=10
x+y=70

Solving the system: x=30, y=40

PLugging in:

30C+40D=600

3C+4D=60

Any value of D that is a multiple of 3 works here.

D=3, C=16
D=6, C=12

are two examples.

Ans:
E

These types of problems can go either way. If the final equation had been 3C+4D=10, Then it would have to be D=1 and C=2. So I would abandon any ideas you have about how these things usually work out.

If there's a faster way to do it than just working it out completely, I don't see it.