n = d+30 n+d = 90 solve for n and d n = 3(d-x) plug in n and d, then solve for x
n = d + 30, take it into n + d = 90 = d + 30 + d = 90
=> = d = 60
d = 30
So n = 90 - d = 90 - 30 = 60
n = 60
60 = 3(30 - x)
30 - x = 20
x = 30 - 20 = 10