Mary tells us that the interview she gave to The National Geographer’s Magazine was published on her 36th Birthday. From the cover of The National Geographer’s Magazine you know that Mary’s interview was in Volume XLVII. You can use the Roman numerals chart on the wall to work out that XLVII is 47.
You know the copy Mary gave to you is Volume XLIX (49) and is dated December 1925. The cover also tells you that National Geographer’s Magazine is published on the 29th of every month, so Volume 47 must have come out on the 29th of the month but 2 months before Volume 49 (29th October 1925). Counting 36 years back from 1925 you can work out she must have been born on 29th October 1889.