Is any type of form of solving systems more useful at certain times?

Which of the following expressions are equivalent ?

Apackage of five erasers cost 0. $.39 at this rate how many how much would 60 erasers cost in dollars and cents

Gravel is being dumped from a conveyor belt at a rate of 25 ft3/min, and its coarseness is such that it forms a pile in the shape of a cone whose base diameter and height are always equal. how fast is the height of the pile increasing when the pile is 14 ft high? (round your answer to two decimal places.) ft/min