Billy was folding laundry. She folded 40 shirts from 7:20 to 7:40. When she finishes folding all the shirts, she was done. (a) If she started folding shirts at 7:15 and ended at 7:39, estimate how many shirts she would have folded in all. Explain your reasoning.

(b) Write an equation to represent how many shirts Billy has left to fold if she started at 7:15. Explain your reasoning.

(This isn't a question you can pick A or B, you have to answer both)