13.032. In 3.5 hours of operation, one stamping press can produce $42\%$ of all ordered parts. The second press can produce $60\%$ of all parts in 9 hours, and the work speeds of the third and second presses are in the ratio of $6:5$. How long will it take to complete the entire order if all three presses work simultaneously?
Solution.

Let $x$ parts be the entire order. Then the working speed of the first press is $-\frac{0.42 x}{3.5}$ parts/hour; the second press is $\frac{0.6 x}{9}$ parts/hour; the third press is $\frac{6}{5} \cdot \frac{0.6 x}{9}$ parts/hour. Therefore, all three presses working simultaneously will complete the entire order in $\frac{x}{\frac{0.6 x}{9}+\frac{0.42 x}{3.5}+\frac{6}{5} \cdot \frac{0.6 x}{9}}=3.75$ (hours) $=3$ hours 45 minutes.

Answer: $i n ~ 3$ hours 45 minutes.