Addition and subtraction are probably the most basic forms of mathematics. You learned how to add and subtract numbers years ago but you still need to practise in KS3 Maths. The only difference is that the numbers you will be adding and subtracting are bigger than before.
As well as dealing with larger numbers, you may also face decimals. When adding or subtracting numbers using a written method, it is important to line up the place values to avoid mistakes with decimals. The place values must line up one on top of the other. If you are adding or subtracting a whole number with a decimal number, be sure to add a decimal point followed by as many 0s as necessary to make the columns line up.
I'm sure you're already a master of addition and subtraction. Just to help you practise the subject, try this quiz. You may find it easy but be sure to read each question carefully. You wouldn't want to make any silly mistakes now would you?