let

g = the number of green cubes, g is integer, g=>0

b = the number of blue cubes, b is integer, b=>0

there are 10 cubes in the bag

g + b = 10

there are 2 times as many green cubes as blue cubes

g = 2b

by solving the system of equations

g + b = 10

g = 2b

we find

b = 10/3

g = 20/3

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since 10/3 and 20/3 are not integers follows there is no solution to the problem.