Cori,

Are you sure you have correctly reproduced this problem?

Suppose the number of red balls is r, the number of blue balls is b and the number of green balls is g. Then from the problem you know the following.

There are 30 marbles in a bag.
r + b + g = 30

There are twice as many red, as blue.
r = 2b or b = r/2

There is 1 more green than red.
g = r + 1

Substituting the second two facts into the first equation I get

r + r/2 + r + 1 = 30

Multiply both sides by 2 to eliminate the fraction

2r + r + 2r + 2 = 60

thus

5r = 58 and r = 58/5.

Thus the number of red balls is a fraction which is impossible.

Penny