Getting to know pairs of numbers which total 100 is quite tricky and often will be ‘worked out’ from other knowledge. However, some certainly can be learnt, including these pairs which all have 5 as the units.
Watch out for a common mistake when doing this. Many children will make the answer 10 too many, for example saying that 65 needs to be added to 45 to make 100.
Again, if the answers are not known the easiest way to work them out is to count on.