Jawabanmu

2014-04-19T19:08:37+07:00
Aku bantu dikit, yg no.4 ya :)
Pr in all cases = (number of combos meeting the criterion) / (total combos) 

denominator in all cases = (20+15+12) c 3 = 47c3 = 16215 

The numerators are: 
1) All different: 20 * 15 * 12 = 3600 
2) all chocolate 20c3 = 1140 
3) all the same (20c3 + 15c3 + 12c3) = 1140 + 455 + 220 
4) all not chocolates 27c3 = 2925 

Divide each of those by 16215 to get the probabilities.