Here is a great phone app that will help you with your arithmetic so you don’t need to be afraid when you are faced with that non-calculator exam. It’s called Maths Tricks and shows you lots of short cuts to performing calculations and gives you endless practice to improve your speed and accuracy. Best of all it’s free!
You can find it on Google Play. I am recommending this because it is good, not because I have any connection to the app.
If you would rather use pen and paper there is a paper version here. Check your answers using the interactive version.
You need to find the lowest common multiple when finding common denominators to add fractions. Highest common factors are also used in fraction or ratio calculations when you cancel down. This interactive worksheet will show you how to work them out.