This is a follow up page to an earlier worksheet on adding decimals mentally. The approach is probably the same as adding 2-digit numbers, in that most people seem to do these ‘in their head’ by adding the units first and then adding on the decimals.
So, with 4.6 + 3.5 my thinking would probably go along the lines of:
‘4 + 3 is 7, making 7.6 plus point 5 which is 8.1’.
It is not the only approach, but is usually a very effective one.