Mustafa’s horse ate 6,400 pounds of hay last year. how could mustafa determine the number of tons of hay his horse ate last year?

Multiply by a relationship that cancels out the units you don't want and introduces units you do need for the answer

6400lb(ton/2000lb)=3.2 tons

This is always the proper way to do dimensional analysis.  Always use unit labels to keep things clear, it may seem silly, but it really makes even the most complicated conversions quite clear and obvious.  Just remember that unit labels "cancel" out just like numbers and everything becomes quite easy.

C, e, and a (hope it )

It would take her 20 minutes.