PrepShorts · Teaching notes · Class 7 Mathematics · Chapter 8, Working with Fractions
Chapter 8 · Working with Fractions
A whole number times a fraction, read as repeated distance
This video could not be loaded. Reload the page to try again.
Sign in with Google9 min.
Keep your place in this chapter — sign in, it’s free.Sign in
What to assume they know
- A fraction as a number of equal parts of a whole, and where it sits on a number line
- Whole-number multiplication read as repeated addition
- Adding fractions with the same denominator
- Mixed fractions, and rewriting one as a single fraction (Class 6, reinforced in Part I, §8.1, p.176)
- Speed as distance per hour, informally: Answering "could this possibly fit?" by estimating in stages for the habit of reasoning about a quantity in stages
What they should be able to do
- Explain why a fractional speed does not change how a distance is computed
- Compute a whole number times a fraction by repeated addition, and say what makes that reading available
- Compute a fraction times a whole number by dividing the whole number into equal parts and then collecting some of them
- Identify the multiplier and the multiplicand in a written product, and say which one gets cut and which one does the cutting
- Use the multiplier's denominator to fix how many equal parts to make, and its numerator to fix how many of them to keep
- Rewrite a mixed fraction as a single fraction before multiplying, and say why that step is done first
- Recognise, from
1/5 × 3 = 3/5, that a product can be smaller than the number being multiplied
Where it usually goes wrong
- "You cannot multiply by a fraction, because you cannot add something a fractional number of times." This is a fair objection to repeated addition, and the chapter answers it by changing the procedure rather than the meaning: for a fractional multiplier you cut the multiplicand and collect pieces. The situation — how far in this much time — is what stays fixed and decides what the answer must be.
- "Multiplying makes a number bigger."
1/5 × 3 = 3/5breaks it on p.174, before the chapter ever raises the question. - "The number under the multiplier's bar is something you multiply by." It is something you divide by: 5 in 2/5 says cut into five, and 2 says keep two. The p.176 arrow diagram shows the two steps in that order.
- "3 × 1/4 and 1/4 × 3 are different problems." In this section they really are different procedures — repeat three quarters, versus cut 3 into four — and that is worth showing. They are not different numbers, which the chapter settles separately on p.186.
- "One and a quarter means one times a quarter." The chapter rewrites 1¼ as 5/4 before doing anything else, on p.176. Show the rewriting as its own step.
- "A fractional speed is a special case needing a special formula." The chapter's whole opening move is that it is not.
Questions to check understanding
- Compute a whole number times a fraction and convert the result to a mixed fraction (the printed exercise on p.177 is exactly this, four times)
- Compute a fraction times a whole number and say which number was cut and into how many parts
- Given a word problem, decide whether it is asking for repetition or for sharing, and write the product accordingly
- Multi-step rate items: given a per-day or per-hour amount, find the amount for a fractional or multi-day stretch (the milk, canal, oil and Moon items on pp.176–177 are all this shape)
- Rewrite a mixed fraction as a single fraction as a first step, and be marked on that step
- Explain, in words, why a fractional speed does not change the method
Examples worth working on the board
- Aaron's five hours (Part I, §8.1, p.173). Inputs: 3 km in 1 hour, 5 hours. The chapter writes the answer out as five 3s added, then as 15 km. Use the addition line — it is the only place in the topic where the repeated-addition reading is shown in full.
- The tortoise's three hours (Part I, §8.1, pp.173–174). Inputs: 1/4 km in 1 hour, 3 hours. The chapter writes three quarters added, giving 3/4 km. Note: the printed sentence that this changes nothing about how the distance is worked out is the hinge of the whole section.
- The number line on p.173. Checked against the printed page. A line from 0 with tick marks a quarter apart; a short arrow spanning the first tick is labelled as the distance in one hour, and a long arrow beneath spans three of them, labelled as the distance in three hours. The 1/4 km label sits under the first interval.
- One-fifth of an hour (Part I, §8.1, p.174). Inputs: 3 km in 1 hour, 1/5 hour. The chapter divides 3 km into 5 equal parts and reads off 3/5 km, then records
1/5 × 3 = 3/5. Checked against p.174: the number line runs from 0 to a labelled 3 km with four interior ticks, and the first of the five intervals is bracketed as the distance in a fifth of an hour. - Two-fifths of an hour (Part I, §8.1, p.175). Inputs: the 3/5 km just found, and the observation that 2/5 is twice 1/5. The chapter multiplies 3/5 by 2 to get 6/5 km, and records
2/5 × 3 = 6/5. Checked against p.175: the same 0-to-3 km line, now with a second bracket covering two of the five intervals. - The labelled callout (Part I, §8.1, p.175, repeated p.176). Checked against the printed page. Two speech-bubble labels point at
2/5 × 3: Multiplier at the fraction, Multiplicand at the 3. The p.176 version adds two curved arrows showing 3 ÷ 5 = 3/5 first, then 2 × 3/5 = 6/5. - Example 1 (Part I, §8.1, p.176). Inputs: 5 grandchildren, 2/3 acre each. The chapter writes five thirds-pairs added and gives 10/3. This is the repeated-addition procedure applied to a fractional multiplicand.
- Example 2 (Part I, §8.1, p.176). Inputs: ₹8 for one hour, 1¼ hours. The chapter first rewrites 1¼ as 5/4, then computes 5/4 × 8 by taking 8 ÷ 4 = 2 and then 5 × 2, reaching ₹10. Note that the chapter divides the multiplicand by 4 before multiplying by 5 — the same two-step order as the number-line work, not the "multiply then simplify" order.
- Figure it Out, p.176 and p.177. Inputs only, no answers: Tenzin drinks half a glass of milk a day (asked for one week, and then for all of January); a team builds 1 km of canal in 8 days (asked for one day, and for a five-day week); Manju and two neighbours share 5 litres of oil three ways (asked for one week and for four weeks); the Moon sets 5/6 hour later each day, starting 10 pm Monday (asked for Thursday); and four products to convert to mixed fractions — 7 × 3/5, 4 × 1/3, 9/7 × 6, 13/11 × 6.
Figures to have open
- A number line that can carry unit ticks, sub-ticks at quarters and at fifths, and two bracket-arrows above and below it. Standard schematic, but it must be able to reproduce both printed arrangements: 0 with quarter ticks (p.173) and 0 to 3 km with five equal intervals (pp.174–175).
- A horizontal bar that can be cut into equal pieces and have pieces lifted out, for sections 5 and 6.
- The multiplier/multiplicand callout, redrawn — the two labels must point at the correct halves of the written product.
- The two-arrow flow of the p.176 diagram: divide, then multiply.
- No photograph or data table from the textbook is needed. The chapter's opening illustration of a boy with a dog and a tortoise is decoration and carries no mathematics.
Where this sits in the book
- NCERT Ganita Prakash, Class 7, Part I, printed Chapter 8 "Working with Fractions", §8.1 "Multiplication of Fractions", pp.173–177 — Aaron and the tortoise with the two number lines (pp.173–175), the Discussion block naming multiplier and multiplicand and setting out the two steps (pp.175–176), Examples 1 and 2 (p.176), and the Figure it Out set that closes on p.177
- Same part, §8.1, p.186, for the order-independence this topic deliberately leaves open
- Same part, SUMMARY, p.198, for the chapter's own compressed statements
- Solutions appendix bound after p.202 in the cached PDF (not part of the printed book), consulted only to check that the exercise inputs above are stated correctly