3
Round to the nearest ten
Discover Mathematics P2: Term 2, Lessons 71–80
GoalRound a number to the nearest ten and use it to estimate.
New words
round · to change a number to the nearest tenestimate · a careful guess that is close to the answerfront-end estimation · use only the tens digit, and make the ones zero
Explanation
To round to the nearest ten, find the two tens that are next to the number and see which is closer. 34 is between 30 and 40 and is closer to 30, so 34 rounds to 30. 37 is closer to 40, so it rounds to 40. If the ones digit is 5, the number is halfway, and we round up: 35 rounds to 40. Rounding makes numbers easy to add in your head. Front-end estimation is another way: keep the tens and make the ones zero. To estimate 33 + 52, use 30 + 50 = 80. The exact answer is 85, so 80 is close.
Worked examples
34 is closer to 30 than to 40, so 34 rounds to 30.
67 is closer to 70 than to 60, so 67 rounds to 70.
Front-end estimate of 33 + 52: 30 + 50 = 80.
Class activity
Rounding hop. Draw a number line from 0 to 100 with only the tens labeled. Your partner calls a number such as 47 or 82. Put a small stone on it and say which ten is nearer. Then use rounding to estimate 38 + 41.
Practice
1
Round 23 to the nearest ten.
20, because 23 is closer to 20.
2
Round 78 to the nearest ten.
80, because 78 is closer to 80.
3
Which ten is closer to the dot, 20 or 30?
30, because 26 is closer to 30.
4
Round 45 to the nearest ten. Explain.
50. The ones digit is 5, which is halfway, so we round up.
5
Use front-end estimation for 41 + 27.
40 + 20 = 60. (The exact sum is 68.)