Once in a while, you may get a question on a numeracy test that looks absolutely impossible – a complicated sum with insanely detailed numbers, for example, almost as if it’s designed to make you panic. Make sure you read the question carefully: almost always, they ask you to give an approximate answer, which is a much, much easier thing to do.

Round off with decimal places

A decimal point is a dot somewhere in a number. You use one almost every time you write down an amount of money, such as £19.50. The dot tells you where the whole pounds (or whole numbers) end and the pennies (or parts of a whole number) begin.

Decimal places are simply how many numbers there are after the dot. Money is normally limited to two decimal places, but in other contexts you may get more decimal places.

A very common question in numeracy tests is to round a number to a certain number of decimal places, which means to find the number of the right size that’s the nearest to the exact answer. Here’s how:

  1. Find the dot in the number and count that many spaces to the right of it. Draw a vertical line here.

  2. If the number to the right of the line is small (from 0 to 4), just cross out everything to the right of the line.

  3. If the number to the right of the line is big (5 to 9), you have to round up. Replace the last number to the left of the line with one number higher.

If the number to the left of the bar is a nine and you have to round up, you have to add one to the next digit to the left as well. If that happened to be a nine as well, you’d have to add one to that and so on. So if you had to round 37.997 to two decimal places, the answer would be 38.00.

If you have to round 43.254 to two decimal places, you count two spaces to the right of the dot and draw a line: 43.25|4. The next number is small, so you just chop off everything after the line to get 43.25.

To round 84.05 to one decimal place, you count one space to the right and draw in a line: 84.0|5; the next number is large, so you replace the 0 with a 1 to get 84.1.

If you have to round a number to the nearest whole number, you use a very similar method – you just put the line where the dot is. For example to round 45.34 to the nearest whole number, it would become 45|34; the three is small, so you would round it down to 45.

To round to the nearest ten, you put the line one space to the left of the dot, just after the ‘ten’ part of the number.

After you’ve rounded, you add a zero at the end so the number is about the same size as it was before – to round 248 to the nearest ten, it would become 24|8 which rounds up to 25, but 25 isn’t anywhere near 248, so you add a zero to make it 250.

The same kinds of rules apply for rounding to the nearest hundred (you put the line after the ‘hundreds’ digit and add two zeros at the end) or the nearest thousand (the line goes after the ‘thousands’ digit and you add three zeros when you’ve rounded).

Two steps for working in approximations

Getting an approximate answer to a complicated sum is as easy as this two-step recipe:

  1. Round all of the numbers involved as roughly as the question tells you to (for example, it might say ‘to the nearest 10p’ or ‘to the nearest whole number’). If it doesn’t tell you, round it after the first digit.

  2. Do the sum with your nice round numbers. Your answer won’t be as precise, but it’ll be in the right ballpark – which is what they want when they ask for a rough answer.