There are a total of 5+6+4+3+2=20 M&Ms in the bag. Since there are 6 yellow M&Ms in the bag, there's a 6/20 probability that the first M&M will be yellow. Then, there are 5 more yellow M&Ms and 19 more total M&Ms in the bag, so there is a 5/19 chance the next M&M will be yellow. Finally, there are 4 more yellow M&Ms in the bag and 18 total M&Ms, so there is a 4/18 chance the last M&M will be yellow.
Multiplying these together, we have:
(6*5*4)/(20*19*18)
This simplifies to:
1/57
Therefore, there is a 1/57 probability that all M&Ms will be yellow.