Question

Mary brother is five years younger than 3 times her age. the sum of their ages 39. how old is mary

Answer 1

Let's say Mary's brother's age is x and Mary's age is y
(1) x = 3y - 5
(2) x + y = 39

Now if we substitute equation (1) into equation (2) we get:
3y - 5 + y = 39
4y = 44
y = 11

Therefor Mary is 11 years old