Katie is twice as old as her sister Mara. The sum of their age is 24.

[math_celebrity]

Let k = Katie's age and m = Mara's age.

We have 2 equations:

(1) k = 2m
(2) k + m = 24

Substitute (1) into (2)
(2m) + m = 24

Combine like terms:
3m = 24

Divide each side of the equation by 3 to isolate m
m = 8

If m = 8, substituting into (1) or (2), we get k = 16.