Let c be the cost of each calculator when shipment was bought.

Let x be the number of calculators in the shipment

Of x calculators, only (x-2) were sold.

Setting up the equation:

(x-2) * (c+5) = 120 + 300

Since c = 300/x

300 - 2*300/x + 5x -10 = 120 + 300

-2*300/x + 5x = 130

In order to save time, substitute the answer value randomly. Start with E, it clicks. So the answer is 30 (E).

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http://gmatclub.com/forum/countdown-beginshas-ended-85483-40.html#p649902