Question 175615
Call the number of nickels n.  Then she returned n+9 dimes and (n+9)+21 quarters.  The dollar amounts of each can be represented by 0.05n, 0.10(n+9) and 0.25((n+9)+21). So you need to solve

0.05n + 0.10(n+9) + 0.25((n+9)+21) = 10.00