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

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