The cost of two books differs by $8. If one of the books costs X the other costs X + 8.
Let an equal rate of discount of D% be applied to the price of both the books, their price is `(1 - D/100)*X` and `(1 - D/100)(X + 8)`
`(1 - D/100)(X + 8) - (1 - D/100)*X = 3 `
The equation derived above is one with two variables, D and X. This cannot be solved for D.
The discount that needs to be applied is dependent on the initial price of the books and changes accordingly.