Question

write an expression to match thewords Ethan collected 16 seashells.He lost 4 of them while walking

Answer 1

OK, this is an easy question you could use simple subtraction 
16-4
If you don't have that as answer let me know your options :)
hope that helps

Answer 2

You could use subtraction to get the answer. 16-4. The answer to 16-4=12

Ethan has 12 remaining seashells