Cristian is 3 times as old as Marie. Marie is 12 yrs. younger than Cristian. How old is Cristian?
C = Cristian´s age
M = Marie´ age
Cristian is 3 times as old as Marie means:
C = 3 M
Marie is 12 yrs younger than Cristian means:
M = C - 12
Replace C with 3 M in this equation.
M = 3 M - 12
Subtract 3 M to both sides
M - 3 M = 3 M - 12 - 3 M
- 2 M = - 12
Divide both sides by - 2
M = 6
C = 3 M = 3 ∙ 6 = 18
Marie is 6 yrs old.
Cristian is 18 yrs old.