Question 1126967: Alex is 15 years older than Cristina. In 4 years, the sum of their ages will be 91. How old are they now? Answer by ikleyn(52787) (Show Source):
A = C + 15 (1) ("Alex is 15 years older than Cristina.")
(A+4) + (C+4) = 91 (2) ("In 4 years, the sum of their ages will be 91")
Substitute expression A = C+15 from eq(1) into eq(2). You will get
((C+15)+4) + (C+4) = 91
2C + 15 + 4 + 4 = 91
2C = 91 - 15 - 4 - 4 = 68 ====>
C = 68/2 = 34.
Answer. C is 34 years old. A is 34+15 = 49 years old.
Check. (34+4) + (49+4) = 38 + 53 = 91. ! Correct !
