Help, what the answer with the step by step?

Which is greater, 7 p 5 or 7 c 5? 7p5 7c5

If 30 lb of rice and 30 lb of potatoes cost ? $27.60? , and 20 lb of rice and 12 lb of potatoes cost ? $15.04? , how much will 10 lb of rice and 50 lb of potatoes? cost?

Interpret the following expression (5y^2-7)^3/(2y)^2