Answer: the expression is c/2 - 4.5
Explanation:
Let c represent the total number of coins that he has initially.
He gives his sister 9 coins. This means that the remaining number of coins that he has is c - 9
He then splits the remaining coins into 2 equal stacks. Therefore, the expression that represents the number of coins in each stack would be
(c - 9)/2
Simplifying further, it becomes
c/2 - 9/2
c/2 - 4.5