So they one of the printer increases at a rate of 1/12 and the other increases at a rate of 1/18. Since you don't know the time it actually takes, you will replace both numerators with and x. (x/12 and x/18). You want to set these up so that they are adding. (x/12 + x/18=1). Since you're adding, you want to change it to the same denominator. The lowest is 36 so you multiply x/12 by 3/3 (so you don't unbalance the equation) and x/18 by 2/2. You'll end up with 3x/36 + 2x/36= 1 which will simplify to 5x/36=1. Multiply each side by 36 to leave the variable by itself. It becomes 5x=36 and when you divide it by 5 you get 7.2. So it's seven and .2 hours, which is equivalent to7 and 1/5 of an hour or 7 hours and 12 minutes.
