31 Ice Cream Flavors at Baskin-Robbins
290 Bones in a Child's Body
2 covers on a book
10 Christmas cards in a set of cards
23 Pairs of Chromosomes (in a Human Body)
I reckon it's likely to be Turn of The Century
2 Ugly Sisters/Stepsisters in Cinderella
122 spaces in a Chinese checker board
300 Length of Noah's Ark in Cubits
6 Correct numbers to win the lottery
You add the two distances together, so the solution is 10m + 5 m = 15m.
1973: Britain Enters the Common Market.