Answer:
Mark is 9 years old.
Explanation:
If Mark Bro is 5 years older and combined is 23, then you want to subtract 5 from 23 giving you 18. Now since there is mark and his brother that is 2. So divide 18 by 2 and you get 9. Mark is 9. Add 5 to the other 9 because mark is 9 and we divided by two so one nine still remains you get (14). Now you can check. Marks age plus his brothers 9+14=23.
Equation: mark=m
m = (23-5)/2
m=18/2
m=9