What is r and how do you solve for it?

Offering 30 if a plus b plus c equals 68 and ab plus bc plus ca equals 1121, where a, b, and c are all prime numbers, find the value of abc. the answer is 1978 but i need an explanation on how to get that.

What numbers are included in the set of integers? what numbers are not included?

You give a $4 tip for a meal that cost $22. approximately what percent of the bill was your tip?