Originally, Zachary had 18 stickers and Andrew had 8.
You can do this by working backwards. Subtract 4 from Zachary’s end total of 13 and add 4 to Andrew’s 13 because Andrew gave Zachary 4. That leaves Zachary with 9 and Andrew with 17. Since this would be after Zachary gave Andrew half his stickers, you know that 9 is half of Zachary’s original amount. So add 9 to Zachary’s to get 18 and subtract 9 from Andrew’s 17 to get 8.
