Question 888050
W + R + B = 55 (I'm assuming there are no other colored marbles)
R = {{{1/2}}}W [he has twice as many white marbles as red marbles]
B = W + 5 [he has five more blue marbles than white marbles.]
Substitute the known values of R and B into the top equation.
W + R + B = 55 
W + ({{{1/2}}}W) + (W+5) = 55
Add
2.5W + 5 = 55
Subtract 5 from each side
2.5W = 50
Divide each side by 2.5
W = 20
If there are 20 white marbles, then there are 25 blue marbles, and 10 red marbles.
20+25+10 = 55