Question

L is 3 times as old as D, who is five yrs older than C. If in 18 years L will be twice as old as c will be then, how old is D?

Answer 1

L = 3D

D = C + 5
L + 18 = 2(C +18)
First eliminate one of the variables. I will use the first two equations to eliminate D.
L = 3(C+5) = 3C + 15
L = 2C + 18
Those two equations tell you, by setting 2C + 18 = 3C + 15, that
C = 3; so L = 24
D = 8.