3a To bake a hundred pancakes, it takes Mom 30 minutes, and Anya 40 minutes. Andryusha is ready to eat 100 pancakes in an hour. Mom and Anya bake pancakes without stopping, while Andryusha continuously eats them. After how much time from the start of this process will there be exactly a hundred pancakes on the table?
First method. Mom bakes a hundred pancakes in half an hour, so in two hours she will bake 400 pancakes. Anya bakes a hundred pancakes in forty minutes, so in two hours she will bake 300 pancakes. Andryusha will eat two hundred pancakes in these two hours. Therefore, in two hours there will be $400+300-200=500$ pancakes on the table. Therefore, for there to be a hundred pancakes on the table, it will take five times less time, that is, $120: 5=24$ minutes.

Second method. Mom's productivity is $100 / 30=3 \frac{1}{3}$ pancakes per minute. Anya's productivity is $100 / 40=2 \frac{1}{2}$ pancakes per minute. Andryusha's productivity in eating pancakes is $100 / 60=1 \frac{2}{3}$ pancakes per minute. Every minute, thanks to the efforts of Mom, Anya, and Andryusha, $3 \frac{1}{3}+2 \frac{1}{2}-1 \frac{2}{3}=4 \frac{1}{6}$ pancakes appear on the table. Therefore, a hundred pancakes will appear on the table in $100: 4 \frac{1}{6}=24$ minutes.

## Answer

In 24 minutes