## 75. Math Puzzle $8 / 71$

Fritz and Klaus are making metal parts and need 20 minutes for each part. They build a device in 2 hours that saves $50 \%$ of the production time. How many parts must be produced at least so that the time spent building the device is recovered and an additional 2 hours of free time is gained?
At least $x$ parts must be manufactured.

Time required without fixture: $x \cdot 20$

Time required with fixture including a time gain of 120 minutes: $x \cdot 10 + 120 + 120$

Setting both terms equal, we get $x=24$. Therefore, at least 24 parts must be manufactured.