The idea in brief. Rounding looks at ONE digit, the one just past the precision asked: smaller than 5 keeps the digit in that place, 5 or greater adds one to it, and nothing further right is written. The precision is a promise about how many places were read, so a nought at the end of the answer stays when the precision asks for it, and is not written when it does not.
Lesson video, with the working
Rounding decimals
3 min 24 s
The film teaches 2 questions in turn. Each question shows first; its working is written as the film explains it.
5 and greater than 5: add 1 to the digit in the hundredths place
3.895 = 3.90 (to 2 decimal places)
Ans: 3.90
Example 1
A decimal with 3 decimal places is rounded to 2 decimal places. The result is 6.40. (a) Find the smallest possible decimal. (b) Find the greatest possible decimal.
5 and greater than 5
(a) Smallest possible decimal: 6.395
4 and smaller than 4
(b) Greatest possible decimal: 6.404
Ans: (a) 6.395 (b) 6.404
How to solve it
Find the last decimal place requested, then read the digit immediately to its right.
In 3.895 rounded to 2 decimal places, the thousandths digit is 5, so add 1 hundredth.
Rename 10 hundredths as 1 tenth. The rounded answer is 3.90; keep the zero to show 2 decimal places.
For a range rounding to 6.40, find the smallest three-place decimal that rounds up: 6.395.
The greatest is 6.404. Its thousandths digit is 4, so it rounds down to 6.40.
Check just outside both ends: 6.394 rounds to 6.39, while 6.405 rounds to 6.41.
The next digit decides the rounding. For a range, check both ends and the value just outside each end.
Practice questions with worked solutions
Question 1 is free to try. The other 5 open in the free trial, each with its full working.
6 is greater than 5, so add 1 to the digit in the ones place
7.62 = 8 (to the nearest whole number)
Answer: 8
For parents
How can my child choose the right place when rounding decimals?
Your child rounds a decimal to the degree of accuracy requested in the question.
What your child learns here
Your child rounds a decimal to the degree of accuracy requested in the question. The MOE P4 syllabus names three possibilities: the nearest whole number, one decimal place and two decimal places. The starting point is understanding which two neighbouring values the original number lies between and which one is nearer.
Read the requested accuracy before calculating, so the answer ends at the ones, tenths or hundredths place for the right reason.
Use the digit immediately after the rounding place to decide whether to keep that place's digit or increase it by one.
Apply the rounding convention at a halfway value, and handle an increase that carries across a 9 into the next place.
Keep a final zero when it shows a requested precision, distinguishing the numerical value of the answer from how accurately it is reported.
Recognise that several original decimals can round to the same result, and test possible starting values using the stated number of decimal places.
Does the answer make sense?
Rounded to one decimal place, 3.46 becomes 3.5 because it is 0.04 from 3.5 and 0.06 from 3.4. Your child can compare those two distances to confirm the choice, then check that the written answer has exactly one decimal place.
How to practise these questions
The Basic item rounds a decimal to the nearest whole number. Intermediate practice includes an answer that carries into the next whole number, a greatest possible starting value behind a rounded measurement, and one measurement rounded to two different accuracies. Challenge items combine rounding with a digit total or ask for all permitted arrangements of supplied digits that give the specified rounded result. Ask your child to state the requested final place before inspecting the deciding digit. When working backwards from a rounded value, keep the permitted number of decimal places in view and test the proposed endpoint by rounding it forwards. For the digit-clue question, a candidate must satisfy every clue as well as give the required rounded result.
Questions parents ask
Which digit should my child inspect when rounding? First identify the place the answer must end at, then inspect the digit immediately to its right. To round to one decimal place, that deciding digit is in the hundredths place. Asking for both place names keeps the rule connected to the number's value.
Why might the rounded answer end with a zero? A final zero can communicate the requested accuracy. An answer written as 4.0 shows one decimal place, while 4.00 shows two. Those forms have equal values, but each responds to a different instruction about precision. Your child should keep the form asked for.
Can my child round in stages to get the final answer? Round directly from the original number to the requested place. Rounding to a finer place first can change a nearby value into a halfway value and affect the next decision. Your child can keep the original number visible while deciding, then write the rounded result only after identifying the relevant digit.
