9.8.7! is equivalent to:

9x8x7
72 x 5,040
504

A] The expression of the number of $20 will found as follows:
given that the number of $10 is x, 
The total number of denominations is 22, then the number of $20 will be given by the equation:
22-x

b] The total number of each denomination will be evaluated as follows:
total amount of money will be given by the equation:
10*x+20(22-x)=300
solving for x:
10x+440-20x=300
10x-20x=300-440
-10x=-140
thus
x=-140/(-10)
x=14 
thus the number of $10 denominations was 14 and the number of $20 denominations was 22-14=8