.
N = # of nickels.
D = # of dimes.
Then the number of quarters is N + D + 10.
Equations:
N + D + (N+D+10) = 40, (1) (coins)
5N + 10D + 25(N+D+10) = 725 (2) (cents)
I just explained to you how to solve the problem:
- write the system in the standard for;
- simplify it;
- then apply the substitution method.
I assume that you are smart enough (by solving the problems of the Sherlock level !), so I hope you will be able to complete it on your own.
The links to the solutions of previous similar problems:
https://www.algebra.com/algebra/homework/coordinate/word/Linear_Equations_And_Systems_Word_Problems.faq.question.1074575.html
https://www.algebra.com/algebra/homework/coordinate/word/Linear_Equations_And_Systems_Word_Problems.faq.question.1074575.html
and
https://www.algebra.com/algebra/homework/coordinate/word/Linear_Equations_And_Systems_Word_Problems.faq.question.1074578.html
https://www.algebra.com/algebra/homework/coordinate/word/Linear_Equations_And_Systems_Word_Problems.faq.question.1074578.html