Let's write a system of equations. We will use D for Derek's age and S for Sarah's age:
D = 4S
(D - 2) + (S - 2) = 36
We can simplify the second equation:
D + S - 4 = 36
D + S = 40
Now we can use substitution to plug 4S in for D in the second equation:
(4S) + S = 40
5S = 40
S = 8
Sarah is 8 years old.
