a store bought some calculators for $400 wholesale and sold all but 10 of them retail for $280 total. The store paid a whole number of dollars for each and received a whole number of dollars each week. If the store made a profit on the sale of each calculator how many calculators did the store originally have?

Answer 1

Number of calulators: n
Price payed for each: p
Price received for each: r

Given:
p, r are integers. r > p
n p = 400
(n-10) r = 280
.:
(40/p -1)r = 28
.:
r = 28p/(40-p)

Find the positive integer solution when r>p.
Then use: n = 400/p