Mental strategies for addition and subraction
 

Relating additions and subtractions to doubles

Teaching Point

Some prerequisite skills for being good at mental strategies for addition and subtraction are: doubling single-digit and two-digit numbers; adding and subtracting multiples of 5; making a number up to the next ten or next hundred by adding on. Focus on these in oral and mental lesson-starters.

Sometimes in additions and subtractions we can exploit the fact that most people are fairly confident with the processes of doubling and halving. Even quite young primary school pupils will quickly show confidence with doubles and will often relate the calculation of ‘near-doubles’ to these. For example, many 5-year-olds would think of 6 + 7 as ‘double 6 and 1 more. This is another example of compensation, of course, turning the calculation into what most of us would find to be an easier one.

Using our facility for doubling with larger numbers could lead us to look at, for example, 36 + 37 and think ‘double 36 and 1 more’. Here are some more examples of how we might use our confidence with doubling and halving to get us started on some additions and subtractions:

48 + 46 could be related to double 46

46 + 46 = 92, so 48 + 46 = 92 + 2 = 94

62 + 59 could be related to double 60

60 + 60 = 120, so 62 + 59 = 120 + 2 – 1 = 121

54 – 28 could be related to half 54 (27) or double 28 (56)

54 – 27 = 27, so 54 – 28 = 27 – 1 = 26

56 – 28 = 28, so 54 – 28 = 28 – 2 = 26