Addition in the Real World
Activity: Checking a Bill
Suppose you buy three items costing $24.99, $16.99, and $37.25 from a mail order catalogue, and the postage is $3.50. Round these prices to the nearest dollar and work out an estimate for the total bill in your head, or in your math notebook if you prefer. Then check your estimation by writing out the problem without rounding and performing the calculation again to give the accurate bill.
Remember to follow the pattern above. Find estimates for $24.99, $16.99, $37.25, and $3.50, then add them up in your head or on paper. Next, write down the exact numbers in a vertical stack, lining up the decimals and place values.
You may have your own method of working things out in your head that works fine for you. If so, there is no need to change how you do this, but you might find the following method useful to work through. The four rounded prices are $25, $17, $37, and $4. To find an estimate for the total cost, add the four rounded prices together.
This is easier to do if you split each number apart from the first one, into tens and units, so 17 can be split into and 37 into . This makes the sum much easier to work out in your head and the sum then becomes .
Then working from the left and adding each number in turn, you can say, “25 and another 10 makes 35; another 7 gives 42; another 30 gives 72; 7 more makes 79, and another 4 gives 83.” The answer is 83, so the total bill will be approximately $83.
To check your estimate on paper, you will need to line the unrounded numbers up and then add them.
You can see that our estimate of $83 was very close to the exact amount $82.73. Now you can be confident in your answer. Good work!