小學雙語數學：Represent multi-step word problems using equations

1. Bill brought a tray with 42 cookies home from his job as a caterer. 
2. He took 18 chocolate chip cookies off the tray. 
3. Half of the remaining cookies are peanut butter, and the other half are oatmeal.
4. Which equation can we use to find the number of peanut butter cookies