This includes two problems/tasks that introduce students to function composition and the necessity of using the correct order. Two situations are given where students must determine which is the better deal: to use a $10 off coupon and then a % off or use the % off first, then the coupon. Students also do an example where they combine two % off coupons.
I did this activity on large whiteboards in groups of 4 students. It only took them about 15 minutes.