Addition, Subtraction and Money Chapter Notes | Year 3 Mathematics IGCSE (Cambridge) - Class 3 PDF Download

Introduction

In this chapter, we learn how to add and subtract larger numbers and how to work with money. We practise regrouping when adding or subtracting, estimating answers, and solving problems in real-life situations. We also learn how to use decimals for money and calculate change when buying things.

Addition

• Add a 3-digit number and a single-digit number, regrouping the ones when necessary.
• Estimate and add a 3-digit number and a 2-digit number, regrouping the ones.
• Estimate and add two 3-digit numbers, regrouping the ones.
• Know how to regroup (exchange) when there are too many ones for the ones place.

Practise addition by:

• Choosing numbers from jars and adding them.
• Reading across or down a grid to form 3-digit numbers and adding a single-digit number to them.
• Solving word problems, e.g. number of people visiting a library or museum.
• Estimating first by rounding to the nearest ten, then adding accurately.
• Applying addition to real-life problems, such as books in a library or cakes baked by a baker.

Think like a mathematician

• Kiko spilled ink on the ones digits of her calculation: 24 + 13 = 381.
• Think of possible calculations that could give this result.
• Find all possible solutions and compare them with other learners.
• Reflect on how to investigate a similar problem next time.

Look what I can do!

• I can add a 3-digit number and a single-digit number, regrouping the ones.
• I can estimate and add a 3-digit number and a 2-digit number, regrouping the ones.
• I can estimate and add two 3-digit numbers, regrouping the ones.

Subtraction

Subtract a single-digit number from a 3-digit number, regrouping when not enough ones are available.

• Estimate and subtract a 2-digit number from a 3-digit number, regrouping when necessary.
• Estimate and subtract a 3-digit number from a 3-digit number, regrouping when necessary.
• Practise subtraction by:
  • Making 2-digit and 1-digit numbers from given digits, then subtracting.
  • Making 3-digit and 1-digit numbers and subtracting, showing all possible results.
  • Solving real-life problems such as damaged milk cartons or melons delivered to a store.
  • Finding out how many parcels are left at the post office after some are taken by vans.
  • Estimating first, then calculating accurately.

Think like a mathematician

• Faisal spilled ink on the ones digits of his subtraction: 49 – 14 = 345.
• Work out possible calculations that could match this result.
• Compare your answers with others and discuss similarities and differences.
• Discuss how this kind of subtraction problem is similar or different from Kiko’s addition problem.

Look what I can do!

• I can subtract a single-digit number from a 3-digit number, regrouping when needed.
• I can estimate and subtract a 2-digit number from a 3-digit number, regrouping when needed.
• I can estimate and subtract a 3-digit number from another 3-digit number, regrouping when needed.

Money

Understand how to use the decimal point to show dollars and cents in the same amount.

• Learn that the dollar part is written before the decimal point, and the cents part after.
• Add amounts of money to find the total spent.
• Subtract amounts of money to find the change after spending.
• Be careful to include the dollar sign ($) and to understand that $5 – 5c is not equal to zero.
• Worked example: $4 and 24 cents is written as $4.24.