Question

A man buys a number of pens for rupees 80 if he had bought 4 more pens for the same amount each pen would have cost him rupees one less how many pens did he buy

Answer 1

let the number of pens be x

Then


80/x - 80/x + 4 = 1

80(1/x - 1/x+4) = 1

80((x + 4 - x)/(x)(x+4)) = 1

80 * 4 / (x)(x+4) = 1

320 = x^2 + 4x

x^2 + 4x - 320 = 0

x^2 + 20x - 16x -320 = 0

x(x + 20) - 16 (x + 20)

(x-16)(x+20)

x = 16 or x = -20

since number of pens cannot be -ve

x = 16

he bought 16 pens