The sum of two numbers if just the numbers added together, so you would just need to do some simple algebra.
The ages added together are 68, and the opposite of addition is subtraction!
(total age) - (sarahs age) = (mothers age)
68 - 24 = 44
44 should be her mothers age!
(If you were trying to calculate the sum of their ages, you would just add them together, and 24 + 44 = 68)
