Both these free worksheets look at questions which involve adding on from one number to make another. In this case they are all adding on from a 3-digit number to the next whole hundred. Again, there is more than one way to do these. As an example look at the question:
“What must be added to 345 to make 400?”
Method 1: add on from 345 to 350 (5) then add 50 to make 350.
Method 2: add on 50 to 345 to make 395 then count on the extra 5.
Plenty of practice still needed at counting up and down in whole tens and as with all mental arithmetic remember to ask your child how they reached the answer as well as seeing whether it is correct or not.