Week 24 · Estimating, fair shares and area · 3 lessons
1
Estimating a total by rounding
Discover Mathematics P3: Term 2, Lessons 91–100
GoalRound numbers to the nearest ten and use them to estimate a total.
New words
round · to change a number to a nearby ten that is easier to useestimate · a careful guess that is close to the exact answerexact · the true answer, with nothing rounded
Explanation
An estimate is an answer that is close but not exact, and it is quick to find. To estimate a total, round each number to the nearest ten and then add the rounded numbers. To round, look at the ones digit. If it is 0, 1, 2, 3 or 4, keep the tens digit the same (round down). If it is 5, 6, 7, 8 or 9, add one ten (round up). So 32 rounds down to 30, and 38 rounds up to 40. Let us estimate 17 + 24 + 33 + 49. The rounded numbers are 20, 20, 30 and 50, and 20 + 20 + 30 + 50 = 120. The exact total is 123, so the estimate is close. An estimate also helps us check: if our exact answer were 223, we would know something was wrong.
Ones digit
What we do
Example
0, 1, 2, 3, 4
round down
32 → 30
5, 6, 7, 8, 9
round up
38 → 40
Worked examples
Round 47 to the nearest ten. The ones digit is 7, so we round up. On the number line 47 is closer to 50 than to 40, so 47 rounds to 50.
Number
Nearest ten
17
20
24
20
33
30
49
50
Estimate 17 + 24 + 33 + 49. Round each number, then add: 20 + 20 + 30 + 50 = 120. The exact sum is 17 + 24 + 33 + 49 = 123.
Mina buys pens for 18 LE, a notebook for 23 LE and scissors for 31 LE. She has 80 LE. Estimate: 20 + 20 + 30 = 70 LE. The exact total is 18 + 23 + 31 = 72 LE, so 80 LE is enough.
Class activity
Estimate and check. Pick four two-digit numbers from a calendar, a book or a price list. Round each to the nearest ten and add to make an estimate. Then add the exact numbers. How far apart are the two answers? Do this three times with different numbers.
Practice
1
Round 64 to the nearest ten. Look at the number line.
The ones digit is 4, so we round down. 64 is nearer to 60 than to 70, so it rounds to 60.
