Carolina,

I would let Mandy's age be M years and Randy's age be R years. I am going to show you how to set up two equations in M and R that you can solve.

The first statement

Randy is 4 times as old as Mandy

tells you that

R = 4 × M.

How old will Randy be in 12 years?
How old will Mandy be in 12 years?

At that time Mandy's age is half of Randy's. That's the second equation.

Solve for M and R.

Don't forget to check your answer.
Penny