A Manager Wants to Analyze the Learning Curve Associated with Producing

Multiple Choice

A manager wants to analyze the learning curve associated with producing one of his company's products. Accordingly, he has gathered the following data: $$\begin{array} { | l | l | } \hline \text { UNIT PRODUCED } & \text { TIME REQUIRED } \\\hline \text { Second } & 5 \text { hours } \\\hline \text { Fourth } & 4 \text { hours } \\\hline\end{array}$$
Approximately how long did it take to produce the first four units?

A)  15 hours
B)  16 hours
C)  18 hours
D)  20 hours
E)  21 hours