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In this activity, we will be looking at how to round numbers to the nearest 100 in order to estimate the answer to a multiplication.
Rules for rounding to the nearest 100
If the tens digit is less than 50, the number is rounded down.
If the tens digit is 50 or more, the number is rounded up.
Estimate the answer by rounding the numbers to the nearest 100 before adding.
We can ignore the 'ones' digit when rounding a 3 digit number to the nearest 100.
744 would be rounded down to 700.
751 would be rounded up to 800.
8,264 would be rounded up to 8,300
Example
242 x 164 = ?
Round each number first
242 
200
164 200
Now calculate: 200 x 200 = ?
2 x 2 x 100 x 100 = 4 x 10,000 = 40,000
Top tip!
When multiplying large numbers with zeros, remove the zeros first, multiply the remaining digits and then add the zeros back!
200 x 200 Remove the four zeros: 2 x 2 = 4
Add the four zeros back on: 40,000
Easy, isn't it?!
Let's get started then!
