The temperature fell from 0 Degrees Fahrenheit to 15 and one-half Degrees Fahrenheit below 0 in 5 and three-fourths hours. Wen tried to find the change in temperature per hour. Her work is shown below.

Negative 15 and one-half divided by 5 and three-fourths = negative StartFraction 31 over 2 EndFraction divided by StartFraction 23 over 4 EndFraction = negative StartFraction 31 over 2 EndFraction times StartFraction 23 over 4 EndFraction = Negative StartFraction 713 over 8 EndFraction

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.