This reduces nicely if you first notice that 16/81 = (2/3)^4.
Then use the properties of powers (of positive real numbers):
(16/81)^(3/4) = ((2/3)^4)^(3/4) = (2/3)^3 = 8/27
10 dimes
The $2 are exchanged for 10 dimes and 4 quarters, which make 14 coins total.
A^2 - b^2 = (a-b)(a+b)
To test it, expand the right hand side.
