Singapore Math Guru

Rounding Off Whole Numbers

A number of students, in our experience, has an issue with the rounding off numbers. This led me to think that the rounding of whole numbers to the nearest thousands, hundreds and tens is not as easily grasped by our weaker students from mid primary level onward in our Singapore math syllabus. What precedes the ability to round off numbers are the basic fundamental skills such as the understanding of the place value chart.

Math education in Singapore is a cumulative build up from primary one. It stands to reason that to conquer what comes after, one should have a firm foundation of the preceding topics in general.

This article aims to address how to help students round off whole numbers with ease.

Let us start off with rounding numbers to the nearest ten.

Round 34 to the nearest ten.

Since we are asked to round 34 to the nearest ten, we would have to look at the digit in the ones place for some direction here. If the digit in the ones place is 5 or greater, we should round the number up. If the digit in the ones place is 4 or less, we should round down. Since there are 4 ones, we must round 34 down. However, the question is round down to what? I suppose this is the question that many students are trying to answer in their heads.

Should we round down to 33? Should we round down to 30?

Well, we round down to 30 and not 33 because we have to round it to the nearest ten. The nearest ten, which is 30, which is equivalent to 3 tens, is the answer.

Hence, 34 when rounded off to the nearest 10 is 30.

Should we round up?

No. And that is because the digit in the ones place is not 5 or more.

Always offer a complete explanation when teaching a student the mechanics of rounding off. That complete explanation offers advice as to what  should be done and what should not be done. It answers the 5 W’s and the 1 H. That is a complete explanation.

On that very front, let us examine an example where we have to round up.

Round 75 off to the nearest ten.

Since the question is asking us to round 75 off to the nearest ten, we look at the digit in the ones place, a degree less than that of the tens place. This is the guiding principle. Next, since the digit is 5 or more, we would have to round up. If the digit is 4 or less, we round down.

What number do we round it up to?

Do we round it up to 76 or 77?

Or do we round it up to 80.

We would have to round it up to 80 which is equivalent to 8 tens. If we are asked to round numbers off to the nearest tens, we have to make sure that the answer is a multiple of ten, in this case, 8 tens.

Then we have to look at 3 digit numbers. Let us try an example of 2 here.

Round 247 off to the nearest ten.

Once again, since the question is ask the student to round the number off to the nearest ten, we look at  the digit in the ones place, one degree less than the tens place. Since 7 is in the ones place, there are thus 7 ones.

We have to round this up to the number that is a multiple of ten. The nearest of which is 250. The answer is hence 250.

And another example.

Round 153 off to the nearest ten.

We now look at the digit in the ones place. If the digit in the ones place is 4 or less, round down. If the digit in the ones place is 5 or more, round up. Since there are 3 ones, we should round down to a number that is a multiple of ten. And 150 is the nearest. 150 can be thought off as 15 tens.

Therefore, 150 is the answer.

May all students be blessed with good grades!