We know that there are 15 children altogether
We know that the number that voted for B+R is double the number of students voted for G+Y
B+R = 2(G+Y)
Think of two numbers that add up to 15, one number is twice of the other number.
For example, 6 and 12.
12 is double of 6 but 6 + 12 doesn't give 15
So, think again
for example 5 and 10
10 is double of 5 and 5+10 does give 15
So, the number that voted Red and Blue is 10 and the number that voted Yellow and Green is 5