1. If x^2 - y^2=10 and x+y=5, what is the value of x-y?

I've tried to figure it out with substitution/simultaneous equations but I keep getting wrong answers..

The actual answer is 2

2. The average (arithmetic mean) of the test scores of a class of p students is 70 and the average of the test scores of a class of n students is 92. When the scores of bloth classes are combined, the avg. score is 86. What is the value of p/n?