Round whole numbers and decimals, with up to two places, to the nearest whole number, or tenth.
Use rounding to check the answers to multiplication and division problems.
Number Framework Stages 7 and 8.
Calculators
Set a variety of division problems with the same calculator answer yet different rounded answers. For example:
“The supermarket sells seven large tins of peaches for $44. One costs $6.285714286.” Discuss why two decimal place rounding is sensible in this scenario. (Supermarkets charge to the nearest cent.) On the class number line, discuss why the lower number is 6.28 and the higher is 6.29, and add them to the ends of the number line. Now discuss which end 6.285714286 is nearer to; 6.28 or 6.29? So conclude $6.285714286 ≈$6.29 (to two decimal places).
Give the students a copy of their own empty number lines and work through the following problems. Discuss the answers carefully.
“Jane has 44 litres of milk to share among seven families. How much does she measure out for each family?” (6.285714286 litres ≈ 6.286 litres to the nearest millilitre or 6.285714286 litres ≈ 6.29 litres to the nearest hundredth of a litre. More accuracy than this is impractical.)
“The market gardener sends 44 tonnes of potatoes to seven supermarkets. How much does he send to each?” (Here sensibly round to the nearest kilogram. So 6.285714286 tonnes ≈ 6.286 tonnes = 6286 kilograms.)
“The service station sells seven large pizzas for $44. What does a pizza cost?” (Assume the service station charges to the nearest 10 cents. So $6.285714286 ≈ $6.30.)
“John shares 44 hard lollies among seven children. How many does each child get?”
(You cannot cut up hard lollies, so 6.285714286 lollies ≈ 6 lollies. Discuss how many would be left over and who would get the extra lollies.)
“Joel has 44 pizzas to share among seven people. How many pizzas does each person receive?”
(Here 6.285714286 is very inappropriate as an answer. Everyone receives six pizzas. There are two whole pizzas left over so probably these should be cut into sevenths and everyone gets two sevenths, which equals 0.2857142. So 44 ÷ 7 = 6 is a sensible answer.)
For each of these division problems, create word problems that are solved by the division yet the rounding rules change with the context. 2 225 ÷ 17, 4 567 ÷ 29, 7 888 ÷ 11 ...