Here we have an excellent set of four maths worksheets introducing subtraction of both tens and units. There is no 'decomposition' involved; in other words the units to be taken away are always smaller than or equal to the units of the number.
Interestingly, most adults who do this 'in their heads' start by taking the tens away and then the units, which is the opposite of the way we do it on paper. Certainly starting with the tens when working mentally is a good strategy to use.
Subtracting tens and units (no decomposition)