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Lesson 16 of 19

A fraction of what is left

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6 min

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On Monday you spend 1/5 of your money. On Tuesday you spend 3/8 of what is left. How much of all your money is left?

The whole is 5 rows by 8 columns: 40 cells.
Step 1 of 4
  1. The whole is 5 rows by 8 columns: 40 cells.
  2. Monday takes 1 of the 5 rows. 4 rows are left.
  3. Tuesday takes 3 of 8 columns from those 4 rows.
  4. 20 cells are left: 20/40 = 1/2.

Find what is left first

1/2 of your money is left.

Find what is left first. Then take the fraction of that, not of the whole.

After Monday, 1 − 1/5 = 4/5 is left. Tuesday takes 3/8 of 4/5. 3 × 4 = 12 and 8 × 5 = 40, so that is 12/40 = 3/10. Then 4/5 is 8/10, and 8/10 − 3/10 = 5/10 = 1/2 is left.

There is a shorter way. Tuesday takes 3/8, so 5/8 of what was left remains. And 5/8 × 4/5 = 20/40 = 1/2.

Check with ₹200. Monday takes ₹40, so ₹160 is left. Tuesday takes 3/8 of ₹160, which is ₹60. ₹100 is left, which is half of ₹200.

Slide the Day 2 columns in the lab. Aim to leave the goal amount.

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Try the lab to finish this lesson.