There are 10 students in a class: 7 boys and 3 girls.
If the teacher picks a group of 4 at random, what is the probability that everyone in the group is a boy?

On a string instrument, the length of a string varies inversely as the frequency of its vibrations. an 11-inch string has a frequency of 400 cylces per second. find the frequency of a 10-icnch string.

The graph shown is the graph of which function?

1) prove that 731^3−631^3 is divisible by 100 2) prove that 99^3−74^3 is divisible by 25