How much cake is left for tomorrow?

03/26/2010

A cake is cut into 15 equal parts. Michael ate 1/3 of the cake, Jane 2/5 of the cake, and Liz ate 1/3 from what Jane ate. The rest of the cake is left for tomorrow. What part of the cake is left for tomorrow?

This problem was relatively easy for kids in our math group.

First, kids found how many pieces Michael ate. What is 1/3 form 15? 5.

Then they figured out how much cake Jane ate. For some kids it was difficult to find 2/5. So I posed the question: What is 1/5 of the cake? 1/5 from 15 is 3. Correct.
If we know that 1/5 is 3 what is 2/5? 2 x 3 = 6. Correct.

Liz ate 1/3 from 6 (what Jane ate). 2. Correct

So now we can calculate how many pieces Michael, Jane and Liz ate together. 5 (for Michael) + 6 (for Jane) + 2 (for Liz) = 13.

How many pieces left for tomorrow if we know that Michael, Jane and Liz ate 13 pieces? 15-13 = 2.
What part of the cake is left for tomorrow? 2/15. Correct!