Hello There!
I feel as though you missed out a part of your question but i have already done this so i know the question.
Mandy's age can be represented as m
Sandy's age can be represented as s.

 = 

s = 

After 8 years:

 = 

Equation:
2m + 16 = s + 8
2m + 8 = s
2m + 8 = 

Therefore, m = 16
s = 

s = 

s = 40
Mandy is 16 now
Sandy is 40 now
40 - 16 = 24
Therefore, Caitlin is correct.
Hope This Helps You!Good Luck :) 
- Hannah ❤