Answer:
There 30 - 40 students altogether
There are 20 girls.
There are 10 - 20 boys.
Therefore if each boy brings 15 chocolate bars, there are between 150 and 300 chocolate bars, and
the number of chocolate bars is a multiple of 15.
The chocolate bars are shared among 21 people (the girls
and the teacher).
Therefore the number of bars is also a multiple of 21.
Therefore the number of chocolate bars is a multiple of the
least common multiple of 15 and 21 which is 105. So there
are 105k chocolate bars for some positive integer k, and since
the number of chocolate bars is between 150 and 300,
150 ≦ 105k ≦ 300
Divide all three parts by 105
1.428571429 ≦ k ≦ 2.857142857
So 2 ≦ k ≦ 2
Therefore k = 2
Therefore, there are 105k = 105(2) = 210 chocolate bars.
Since every boy brought 15, there are 210÷15 = 14 boys
Since there are 20 girls and 14 boys, there are
20+14 = 34 students in the class.
Since every girl and the teacher shared the 210 chocolate bars,
every girl and the form teacher received 210÷21 = 10 chocolate
bars.
Solution: 34 students in the class. The form teacher received
10 chocolate bars.