Problem 3. To bake 100 buns, it takes Mother 30 minutes, and Maya 40 minutes. Marko eats 100 buns in one hour. Mother and Maya bake buns continuously, while Marko eats continuously. After how much time from the start of baking will there be exactly 100 buns on the table?
Solution. First method. If the mother bakes 100 rolls in 30 minutes, then in 1 minute she will bake $\frac{100}{30}=\frac{10}{3}$ rolls. If Maya bakes 100 rolls in 40 minutes, then in 1 minute she will bake $\frac{100}{40}=\frac{10}{4}$ rolls. Since the two of them bake together, then in 1 minute they will bake $\frac{10}{3}+\frac{10}{4}=\frac{35}{6}$ rolls. Marko eats 100 rolls in one hour, so in 1 minute he will eat $\frac{100}{60}=\frac{10}{6}=\frac{5}{3}$ rolls. If the time after which 100 rolls will remain on the table is denoted by $x$, then we have $\left(\frac{35}{6}-\frac{5}{3}\right) x=100$, i.e., $x=24$ minutes.

Second method. If the mother bakes 100 rolls in 30 minutes, then in 2 hours she will bake 400 rolls. If Maya bakes 100 rolls in 40 minutes, then in 2 hours she will bake 300 rolls. Marko eats 100 rolls in one hour, so in two hours he will eat 200 rolls. Then in two hours we have $400+300-200=500$ rolls. We need to have 100 rolls left, so $500: 5=100$ and it follows that the required time is $120: 5=24$ minutes.