probability

p(liquorice+mint) = (12/20)*(5/19)

 

p(liquorice+humbug) = (12/20)*(3/19)

 

p(mint+humbug) = (5/20)*(3/19)

 

sum the above to fiind the total probability:  (12*5 + 12*3 + 5*3)/(20*19)

 

I'll leave you to crunch the numbers.