A. order doesnt matter so it's combination C (3,2)=3!/2!=3 possibilities (a,b), (a,c), (c,b) ABC to distinguish different bills with a=1 b=1 c=10. you have 2/3 chance of making 11 versus 1/3 of 2. good deal.
B. Tosses 3 coins, all heads or tails. (1/2)^n for n=3, 1/8 for h, 1/8 for the. 1/8+1/8=1/4 or 25% of getting $30 yet 75% for $2 so not very fair.
C. Assuming number cube is dice. 7=1+6=2+5=3+4
so with two dice, probability of rolling sum of 7 is 1/6 since there are 6 possible ways and 36 outcomes. $50 is a 1/6 chance with a 5/6 chance of getting $2.
personally I would go with B because I like to take risks but A is the safest and most fair consideringredients the fact that you risk getting paid 20% of what u should at least be getting.