G4M5L28 Subtract Mixed Numbers

Meredith had 2 m 65 cm of ribbon. She used 87 cm of the ribbon. How
much ribbon did she have left?

Meredith had 2
m 65 cm of
ribbon. She
used 87 cm of
the ribbon.
How much
ribbon did she
have left?

Do we have enough fifths to subtract 3 fifths?

3 oranges 2 apples – 1 apple is…?

Materials: (S) Personal white board

Solve the problem.

Problem 1: Subtract a fraction from a mixed number by counting back.

3 dogs 2 puppies – 1 puppy is…?

3 ones 2 fifths – 1 fifth is…?

Now do we have enough fifths?

Do we have enough fifths to subtract 3 fifths?

Problem 2: Subtract a fraction less than 1 from a whole number by
decomposing the subtrahend.

Count back 1 fifth on the number line.

That’s 4.

Model on a number line, and then model using arrows.

Lesson Objective:

Subtract a fraction from a mixed number.

•Use Problems 2(a) and 3(c) to compare the different methods to subtract
when there are not enough fractional units.

•How is 7 tens 3 ones – 9 ones like 7 ones 3 tenths – 9 tenths? How is it
different?

•Tell your partner the process of subtracting a fraction from a mixed number
when regrouping is necessary.