How is 209.942 to 1 dp 209.9?

Answer 1

To round a number to one decimal place means we are going to either round the first digit after the decimal point up (add 1 to it) or down (leave it as it is).

To determine whether we round it up or down we have to look at the digit to its immediate right. We then ask, "is this digit equal to or greater than 5?". If it is then we round up, else we round down.

In this example, the first digit after the decimal point is "9". The next digit (i.e. to its immediate right) is "4". As 4 is less than 5 we round down and leave the "9" as it was. Therefore this number is written as 209.9 to one decimal place.

Note: Another way to think about this is that what we are actually asking is whether 209.942 is closer to 209.9 or 210.0 (the answer is that it is closer to 209.9).