This is very similar to the process of adding 19 to a 2-digit number. The easiest way to do this is usually to take twenty and then add one. This mental arithmetic method is much quicker than carrying out the question using pencil and paper methods.If children can grasp these ideas it gives them a great deal of confidence and they can develop their own mental methods of answering other, similar questions; eg subtract 18 by taking 20 and then adding 2, or subtracting 39 by taking forty and adding one.
Tag: subtract
Maths worksheet: subtract from 20 mentally
There are a number of different ways in which subtraction problems can be phrased including terms such as ‘Take from’ and ‘How many more’. This maths worksheet looks at some of them, whilst concentrating on subtracting from 20 using mental methods.
The usual method of mentally subtracting is to ‘add on’. For example 20 – 7.
Add 3 to 7 to make 10 and then add a further 10 to make 20. Answer 3 + 10 which is 13.
Or, the ten could be added first to make 17 and then count on to 20.
Either way is equally good.