Level 3
Irene has enough flour to bake either 21a tarts or 18 biscuits. If she has finished baking 5a tarts and 6 biscuits, how many more tarts can she still bake in terms of a?
Level 2
Lance exercised for a total of (8f + 90) min in a week. For the first 3 days, he exercised for 2f minutes per day. For the next 2 days, he exercised for 30 minutes per day. For how long did he exercise for the rest of the week?