400
Mrs. Long spent $306 on a new phone and 3 phone covers. She spent $243 on the new phone. Each phone cover was the same price. How much did Mrs. Long spend on each phone cover?
A. $189, because 306-243=63 and 63x3=189
B. $21, because 306-243=63 and 63 divided 3=21
What is B?