The temperature fell from 0    to      below 0 in    hours. wen tried to find the change in temperature per hour. her work is shown below.

when she checked the answer by estimating, it did not make sense. what error did wen make?
wen incorrectly converted a mixed number to an improper fraction.
wen added the numerators and denominators instead of multiplying.
wen did not take the reciprocal of the divisor.
wen did not change the division problem to multiplication.