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
I reckon it's likely to be Turn of The Century
23 Pairs of Chromosomes (in a Human Body)
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
1973: Britain Enters the Common Market.
Then A = 31, b = 31 and c = 31/15 = 2.066...
3 Primary Colours - Blue, Red, and Yellow