Rather than repeat the background information, if you have hit on this post for the first time, we suggest that you look at the FP/KS1 Introducing algorithms 1: 'IF' and 'THEN' and FP/KS1 ...
Rather than repeat the background information, if you have hit on this post for the first time, we suggest that you look at the 'FP/KS1 Introducing Algorithms: 'IF' and 'THEN' first.