By year 4 children should be comfortable with 2-digit addition ‘in their heads’. This maths worksheet on adding 19 to any 2-digit number will show whether they are confident and quick with their mental methods.
There are usually two processes involved when adding 19. It is often best done by adding twenty and subtracting one, although some children may well add ten and then add 9. This is easier and quicker than writing the sum down in columns and adding up using the standard method.