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Place Value
Objectives
To round 2-digit numbers to the nearest multiple of 10.
Resources
Abacus Workbook 2 | Prepared shop as in Lesson 3 | Calculator | 📄 Rounding up and down to a ten | 📄 0-100 landmarked line | 📄 100 square
Vocabulary
round up | round down | nearest 10
Today's Lesson
What to Get Ready
Ensure you have these prices for items in the shop as follows:
17p, 25p, 32p, 49p, 55p, 63p, 74p, 86p and 99p.
Introduction
Show your student items priced: 17p, 25p, 32p, 49p, 55p, 63p, 74p, 86p and 99p.
Take each item in turn and round to the nearest 10p. Mark each on the 📄 0-100 landmarked line, having added ‘p’ after multiples of 10.
Discuss how 99p rounds to 100p which is £1; write £1 by 100 on the line.
Explain that if we want to add several of these prices quickly, it is much easier to add, say, 20p, 60p and 50p than to add 17p, 63p and 49p.
Model this by naming some prices and asking your student to round and then add them together. You will need to do this slowly.
What is your student's strategy for adding multiples of 10? Are they using number facts to help, for example, 30 + 40 by using 3 + 4? Or do they use ‘Spider’ to add 10s on the 📄 100 square?
Emphasise that this works well to roughly work out the overall price of a few items, but that in an actual shop we would never round to the nearest 10p or £ and offer that amount of money, as you might end up with prices rounded down. In a real-life situation, you would need to always pay with an amount that is equal to or more than the total price!
Main Activity
Show items priced 27p, 18p and 11p.
Explain to your student that they have got £1 to spend – is this enough to buy all three things?
Ask your student to round all three items to the nearest 10p to get a rough idea.
They should do this to arrive at 30p, 20p and 10p.
Say: ‘What is 30p and 20p? How could we work this out? Now we need to add on the last 10p. What have we got? So the total is about 60p, so we’ve probably got enough money to buy all three things.’
Did your student use ‘Spider’ to help add 10s?
Ask your student to use a calculator to find the exact total, 56p.
Ask: ‘What is 56p to the nearest 10p?’
Explain that this is one example where rounding can be really useful.
This video demonstrates using the 0-100 number line to help with rounding and then adding.
Repeat with different sets of three items for sale.
Complete the sheet 📄 Rounding up and down to a ten.
Workbook
To help your student consolidate their learning from today, please ask them to complete the activities on page 31 of Abacus Workbook 2.