A basket contains some white, red and black ball. There are 15 white and red balls. There are 18 red and black balls. The number of black and white balls is 9. Find the number of balls for each colour .
w+r = 15
r+b = 18
b+w = 9
substitution might be easiest
or, maybe this will look easier
0b+1r+1w = 15
1b+1r+0w = 18
1b+0r+1w = 9
now elimination looks like a good way.