2. let x - no. girls
100 = 30 + x
x = 100 - 30
x = 70 girls
3. let r - red pingpong balls; b for blue
r = 5b > first eqn
60 = b + r > second eqn
substitute r to the 2nd equation
60 = 5b + b
60 = 6b
b = 10
substitute b to the first eqn
r = 5b
r = 5(10)
r = 50
blue = 10 ; red = 50