The local skating rink pays Mary a fixed rate per pupil plus a base amount to work as a skating instructor. She earns $90 for instructing 15 students on Monday afternoon. Last Friday, she earned $62 for working with 8 students. Lisa is also a skating instructor. She receives half the base amount that Mary does, but she is paid twice as much per student. Who would earn more money instructing a class of 20 students?

Mary’s earnings increased by $90 − $62 = $28 when the number of students increased by 15 − 8 = 7, so she earns $28 ÷ 7 = $4 per student. Her base amount is $90 − 15($4) = $30. Lisa’s base amount is half of $30, or $15, and she earns $8 per student, so for 20 students she earns $15 + 20($8) = $175, compared with Mary’s $30 + 20($4) = $110. Lisa earns $65 more, so the correct choice is B.

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