The skateboard that Jose wants costs $90. Jose has a coupon for 1/5 off the retail price. If Jose saves $18 a week, how long will it take to save enough to buy the skateboard?
1/5 off the retail price of $90 is $90 * 1/5 = $18.
With the $18 Jose saves each week, it will take him $90 - $18 = $72 to buy the skateboard.
Therefore, it will take Jose $72 / $18 = 4 weeks to save enough to buy the skateboard.