Let certain number = x

Let original price of 1 pen = R5/x = 500cents/x

hence new price of 1 pen = 500/x - 5 , in (cents).

New number of pens = x+5.

Therefore new price of 1 pen x new number of pens = R5 = 500cents

(500/x - 5)(x+5) = 500

500+ 2500/x - 5x - 25 = 500

2500-5x^2 - 25x = 0

5x^2 + 25x -2500 = 0

x^2 + 5x - 500= 0

(x-20)(x+25) = 0

x = 20 or x = -25 , but x should be positive,

Therefore original number of pens,x, is 20.