Subtracting a decimal number from a whole number can be done ‘in your head’ if it only involves tenths, usually by ‘counting on’ .
A good knowledge of pairs of numbers which make 10 really helps here.
Let’s look at:
1 – 0.3
Probably the easiest way to do this is by ‘counting on’.
Step 1: Count on from 0.3, in tenths up to one.
(eg 0.4, 0.5, 0.6 …etc
which comes to 0.7
1 – 0.3 = 0.7
Of course, if you know that 7 + 3 makes 10, then you can transfer this knowledge ie 0.3 + 0.7 = 1
In the same way:
4 – 3.3 = 0.7
Watch out for the answer 1.7 to the above question which is incorrect and shows a lack of understanding of numbers and ‘counting on’.