Here we have a straightforward mental arithmetic worksheet on subtracting 9 from any 2-digit number. Usually the best way to do this is to subtract ten and then add one. Probably the only time that this is not the best way is if the number to take 9 from also has a 9 in the units.
Knowing this quick technique gives children confidence and they can then move on to subtracting 19, 29, 39….. etc in their heads.