Robots Robert and Hubert assemble and disassemble coffee grinders. Each of them assembles a coffee grinder four times faster than they can disassemble it. When they came to the workshop in the morning, several coffee grinders were already assembled there.

At 7:00, Hubert started assembling and Robert started disassembling, and precisely at 12:00, Hubert finished assembling a coffee grinder and Robert finished disassembling another one. In total, 70 coffee grinders were added during this shift.

At 13:00, Robert started assembling and Hubert started disassembling, and precisely at 22:00, Robert finished assembling the last coffee grinder and Hubert finished disassembling another one. In total, 36 coffee grinders were added during this shift.

How long would it take to assemble 360 coffee grinders if Robert and Hubert were assembling together? (K. Pazourek)

Hint. How many coffee grinders will be added per hour in each shift?
In the five-hour morning shift, 70 coffee grinders were added, which corresponds to $70: 5=14$ coffee grinders per hour. In the nine-hour afternoon shift, 36 coffee grinders were added, which corresponds to $36: 9=4$ coffee grinders per hour. If the robots worked one hour in the morning mode and one hour in the afternoon mode, they would produce $14+4=18$ coffee grinders and produce the same amount of coffee grinders as if they had assembled (and not disassembled) $\frac{3}{4}$ of an hour. The robots would assemble 360 coffee grinders in $\frac{3}{4} \cdot 20=15$ hours, since $360=18 \cdot 20$.

Another solution. If $h$ represents the number of coffee grinders Hubert assembles per hour, and $r$ represents the number of coffee grinders Robert assembles per hour, then Hubert disassembles $\frac{1}{4} h$ coffee grinders per hour and Robert disassembles $\frac{1}{4} r$ coffee grinders per hour. The information from the problem leads to a system of two equations:

$$
\begin{aligned}
& 5\left(h-\frac{1}{4} r\right)=70 \\
& 9\left(r-\frac{1}{4} h\right)=36 .
\end{aligned}
$$

Solving this system yields $r=8$ and $h=16$. Therefore, the two robots would assemble $r+h=24$ coffee grinders per hour. Thus, 360 coffee grinders would be assembled in $360: 24=15$ hours.