The cost per unit of electricity during a 4 year period is as follows: Year 1, $3 per unit; Year 2, $4 per unit; Year 3, $5 per unit; Year 4, $6 per unit. If a factory spent equal dollars on electricity each year, what was the average cost of a unit of electricity during the 4 year period?  Choices.
A) $ 4.31	B) $ 4.51	C) $ 4.21	D) $ 5.21	E) $ 5.61

Rationale: let money spent each year = $ 60 units bought first year = 20 units bought second year = 15 units bought third year = 12 units bought fourth year = 10 total number of units = 57 total money spent = 60 * 4 = 240 average = 240 / 57 = 4.21 answer : c
Answer: C