(This solves to n = 17 meaning there are 17 nickels)
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Explanation:
"There are 25 pennies, 21 dimes, and 29 quarters", so there are 25+21+29 = 75 coins so far. This is the amount of coins that aren't nickels. The rest are nickels, which is some amount n. This adds to the 75 to get 75+n. Set this equal to 92 because there are 92 coins in all. That's how we end up with 75+n = 92.
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Extra info
The equation 75+n = 92 is the same as n+75 = 92. Subtract 75 from both sides to isolate n. You should get n = 17 after doing so.
