Man in Training

Last week my nephew Ethan was here during a homeschool day. Madison was taking a math test and Ethan was helping her read the word problems. He was NOT supposed to be helping her solve them however I did not think that was a problem since she is doing multiplication this year. The two of them read the following word problem together:
Lyle watched seven horses gallop across the field. How many hooves were making tracks?
Just before Madison boldly stated "Twenty-eight!", Ethan says "Wait! How big is the field?"