Question

Question #ac2d1

Answer 1

I got `15` pens at `$6` each.

Explanation:

Call the number of pens `x` and the cost of one pen `y` we get:

At the original price:
`xy=90`

If we subtract `$1` from the original price we can buy `3` more pens or:
`(x+3)(y-1)=90`

so
`(x+3)(90/x-1)=90`
`90-x+270/x-3=90`
`-x^2+270-3x=0`

Using the quadratic Formula we get that:

`x_1=15` (we get a negative solution as well `x_2=-18`).

So the total number of pens is `15` and the cost of one pen will be:
`y=90/x=90/15=$6`