1. In a classroom, there are chairs and stools. A child is sitting on each chair and each stool. Each chair has 4 legs, each stool has 3 legs, and each child has 2 legs. Together, this results in a total of 39 legs and feet.

How many chairs are there in the classroom?
A) 3
B) 4
C) 5
D) 6
E) 9
A1. B) 4

A chair with a child on it has a total of 6 legs and feet. A stool with a child on it has a total of 5 legs and feet. With 7 chairs, we already have $7 \times 6 = 42$ legs and feet, which is too many. We create a table with the number of chairs and the number of legs and feet:

| Number of chairs: | 0 | 1 | 2 | 3 | $\mathbf{4}$ | 5 | 6 |
| ---: | ---: | ---: | ---: | ---: | ---: | ---: | ---: |
| Number of legs and feet: | 0 | 6 | 12 | 18 | $\mathbf{2 4}$ | 30 | 36 |
| Remaining legs and feet: | 39 | 33 | 27 | 21 | $\mathbf{1 5}$ | 9 | 3 |

The remaining number of legs and feet must come from the children sitting on stools. This number must therefore be divisible by 5. The only number in the third row that is divisible by 5 is 15. Therefore, the number of chairs is 4 (and the number of stools is $\frac{15}{5}=3$).