Level 2
Zoe paid a total of $72 for an eraser, a ruler and a pen. The eraser cost 27 of the total cost of the ruler and the pen. The pen cost 15 of the total cost of the eraser and the ruler. Find the total cost of the pen and the eraser.
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?