Granny says "I am $84$ years old - not counting my Sundays". How old is she really?
Granny is calculating her age as if every week has six days not seven - she is therefore saying she is younger than she is because (for example) Granny is working on $6$ weeks only having $36$ not $42$ days but a year is still $365$ days long.
If you liked this problem, here is an NRICH task
which challenges you to use similar mathematical ideas.
This problem is taken from the UKMT Mathematical Challenges.