Problem 3. Martin has three containers, two of which hold exactly $3 l$ and $5 l$, while the third contains $10 \mathrm{l}$ of milk. He needs to measure out exactly $4 l$ of milk using the three containers.

Can Martin do this, and if so, how?
Solution. It can be done. A 10 l can will fill the 3 l can. Then, the milk from the 3 l can will be poured into the 5 l can. Again, the 10 l can will fill the 3 l can. From the 3 l can, the 5 l can will be topped up. In the 3 l can, 1 l will remain. The milk from the 5 l can will be poured back into the 10 l can. The remaining 1 l in the 3 l can will be poured into the 5 l can. Now, the empty 3 l can will be filled and then poured into the 5 l can, so it will have 4 l of milk.