**Hint for process of elimination:** It can help you to draw boxes, where each box represents $latex \dfrac{1}{2} $ hour. Write the number 20 in each box until you get to 150 pictures. Then just count boxes.

It will also help you to think intuitively about this problem. You know how time works, and you know what twenty doggy pictures looks like. Approximate. How many half hour time slots will it take to draw 150 doggies?

Read on for a full solution.