Week 7 · The standard way to multiply multi-digit numbers · 3 lessons
3
Multiplication problems from everyday life
Discover Mathematics P5: Term 1, Unit 3, Lessons 5–7
GoalSolve everyday problems that need one or two multiplications.
New words
rate · an amount for one unit, such as LE for 1 kgtotal · the amount you get when you put all the parts together
Explanation
Multiplication fits stories with equal groups: the same amount again and again. Look for the amount in one group and the number of groups, then multiply. Many problems need two steps, and then it helps to write what each step finds. Read, find the question, plan the steps, estimate, calculate, and check that the answer is sensible for the story, with the right unit (LE, kg, m). For example, a hall has 24 rows with 135 seats in a row. Step 1: seats in one hall = 24 × 135 = 3,240. Step 2: for 3 halls, 3,240 × 3 = 9,720 seats.
✓Tip
Write what each step finds, with its unit, before you start the next one.
Worked examples
A tuk-tuk driver earns 285 LE a day and works 26 days in a month. 285 × 26: 285 × 6 = 1,710 and 285 × 20 = 5,700. Sum: 1,710 + 5,700 = 7,410. He earns 7,410 LE. Estimate: 300 × 25 = 7,500.
A school orders 48 packs of paper with 500 sheets in each pack. Sheets: 48 × 500 = 24,000. Each pack costs 65 LE. Cost: 48 × 65 = 3,120 LE (48 × 60 = 2,880 and 48 × 5 = 240). Two different questions use the same 48 packs.
A hall has 24 rows and each row has 135 seats. 24 × 135 = 24 × 100 + 24 × 35 = 2,400 + 840 = 3,240 seats. A school has 3 such halls, so 3,240 × 3 = 9,720 seats in all.
Class activity
Market maths. Pick an item sold by weight at a market stall, for example oranges at 14 LE per kg. How much do 36 kg cost? How much do 125 kg cost? Make a small price table for 10, 20, 50 and 100 kg and check each row with multiplication.
Practice
1
A bookshop sells 36 books a day at 85 LE each. How much money does it take in a day?
