Algebralization of a puzzle

Question

I tried to put this algerba through algebra and couldn't:

Each of 20 students in a group have solved three problems from the homework assignment, and each problem was solved by two students. How many problems were in the assignment?

Answer 1

First compute, how many tasks where performed by all students together. As each students solved three (different) problems, we have a total of $$20 * 3 = 60$$ performed tasks. Now every problem was solved by exactly two students. So there were at all $$ \frac{60}{2} = 30$$ problems.