Only because 20/21 is a ridiculous answer. Never seen John McCririck tic-tac'ing that one on the Racing Line... 'on the shoulders' 'double carpet' etc... :nono:
What does the bookie do with fractions... for example 21.6666 from the first bet?
Do you round it up/down and then calculate the 2nd bet? or put the fraction (to how many decimal places?) on the 2nd bet and then round the final result up/down??