#20

After working for 4 hours, Frank has made 21 machine parts.
That means that 21 machine parts over the course of for hours (21/4) yields 5 and 1/4 machine parts.
Let 's call X the rate of creation, which is 5.25 (5 and 1/4).

If he continues at the rate of creation (X) for 7 hours, you would calculate his work as X * 7.
That would be either 5.25 * 7 OR 5 and 1/4 * 7, which both equal 36.75, or 36 and 3/4.