Question 115449
Let age of Ricardo is x and age of his sister is y.
Ricardo is 4 years less than twice as old as his sister.
This can be written as x = 2y - 4.
That is, x - 2y = -4-----------(1)
The sum of their ages is 20.
This can be written as x + y = 20----------(2)

Solve (1) and (2).
(1) - (2) gives -3y = -24
That is, y = 8.

Therefore, x = 12. 

Hence, Ricardo is 12 years old.