Answer:
To solve, use a simple one-step equation
Known: Number of coins; Number of Nickles.
Unknown: Number of Dimes.
a) To find the number of Dimes
b) Use x (or virtually any letter) to represent the unknown number of dimes
c) 9+x=13
d) Subtract 9 from both sides to cancel out the 9 and isolate the variable on one side. x=4
e) 9+4=13, so yes, the answer is correct
