Computer Game Sales

[GmatKiss]

Computer Games Plus needs to get rid of its copies of an old computer game. If it lowers the cost of the old computer game by $5 dollars, it can increase sales of the old computer game by 10 units and still generate exactly $100 of revenue from the old game. How many units of the old computer game did Computer Games Plus sell after implementing the new selling strategy?
A) 10
B) 15
C) 20
D) 30
E) 50

[neelgandham]

Computer Games Plus needs to get rid of its copies of an old computer game. If it lowers the cost of the old computer game by $5 dollars, it can increase sales of the old computer game by 10 units and still generate exactly $100 of revenue from the old game. How many units of the old computer game did Computer Games Plus sell after implementing the new selling strategy?
A) 10
B) 15
C) 20
D) 30
E) 50

Let s = no of copies sold, c = cost of one game, s*c =100

If it lowers the cost of the old computer game by $5 dollars, it can increase sales of the old computer game by 10 units

=> Cost of one game = c-5, No. of copies sold = s+10, (s+10)*(c-5) =100

First glance s=10, c=10; Question : [spoiler]s+10 = ? = 10+10 = 20 Answer:C[/spoiler]

If you couldn't get the answer by looking at the equation, solve further

(s+10)*(c-5) = s*c=100
=> sc + 10c - 5s - 50 = sc
=> 10c - 5s = 50
=> 2c - s = 10 - 1

and cs = 100, c = 100/s- 2

From one and (2*(100/s))-s =10, Solving the quadratic equation, you get s =10, [spoiler]and S+10 = 20[/spoiler]

[shankar.ashwin]

You could also solve it from the answer choices.
Start with C

20 games after new strategy, therefore 10 before that.
Total = $100.

Price(old) = 100/10 = $10
Price(new) = 100/20 = $5 (which is $5 lesser than 10) Success.