Eleanor is 11 years and 4 months old. Her sister Mary is 3 years and 10 months younger than Eleanor.
How old is Mary? Give your answer in years and months
I would do this as subtract 4 years, and than add two months. i.e. 7 years and 6 months
Katharine was born on 1st May 1997
Helen was born on 1st March 2003
(a) By how many years and months is Katharine older than Helen?
(b) Sam is exactly 18 months younger than Katharine. Work out sam's date of birth
a) If both dates were May the difference would be 6 years (2003-1997). But there are two months difference between March and May, so subtract those to give 5 years and 10 months.
b) Subtract 2 years, then add six months.
It is worth learning the month -> number correlations so that she can see quickly that October is 10, August is 5, May is 5 etc. Then sums like May - minus two months = March becomes the simpler sounding 5-2=3 (and you'd know 3 is March).