A merchant paid $300 for a shipment of x identical calculators. The merchant used 2 of the calculators as demonstrators and sold each of the others for $5 more than the average (arithmetic mean) cost of the x calculators. If the total revenue from the sale of the calculators was $120 more than the cost of the shipment, how many calculators were in the shipment?
A)20
B)30
C)25
D)35
E)40
The solution is
(x-2) * ( 5 + 300/ x) =420
=> 5x - 600/x - 10 + 300 = 420
=> 5x^2 - 130x - 600=0
=> x^2 - 26x - 120 = 0
Solving we get, x = 30 or x = -4
x cannot be negative as it is the number of calculators
So, there were 30 calculators.
Hence the answer is B.