Adding or subracting whole hundreds ‘in your head’ can cause problems for children. It takes time to realise that if you know that 6 + 7 = 13 then it is easy to work out 600 + 700. Children are thrown by the larger numbers and do not see the patterns involved. eg:
6 + 7 = 13
60 + 70 = 130
600+ 700 = 1300
A good way to help here is to think of 60 as 6 tens and 70 as 7 tens, so 60 + 70 is 6 tens plus 7 tens and so on. The only real mental calculation that is being done is the original 6 + 7. The rest is an understanding of place value.