Ralph van Dijk and side-kick Matrix have more questions to consolidate mental maths skills and the focus today is adding and subtracting two-digit numbers.

Subtraction from a two-digit number A group arrives at an airport to check in their 21 bags. They each want to keep a piece of hand luggage. Unable to work out how many bags they will need to check in ...

My last column introduced the concepts underlying BCD (binary-coded-decimal) representations. In particular, we considered unsigned versus 10s-complement versions of BCD numbers. In this column we are ...