.
            This problem is to be solved using one single equation in one unknown.
            I will show you on how to do it.
            The major step is to choose the leading unknown reasonably.
Let x be the number of quarters.
Then the number of dimes is (x-6), and the number of nickels is 2x.
By knowing it, you can write the "money" equation momentarily
    5*(2x) + 10*(x-6) + 25x = 435   cents.
Simplify
    10x + 10x - 60 + 25x = 435
    45x = 435 + 60 = 495
      x            = 
 = 11.
ANSWER.  11 quarters;  11-6 = 5 dimes,  and  2*11 = 22 nickels.
CHECK.   5*22 + 5*10 + 25*11 = 435 cents,  which is precisely correct (!)
Solved.
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The lesson to learn from my post is  THIS :
    This problem can be solved using one single equation in one unknown.
    It is designed and intended to be solved in this way.
    It is intended for 6-7 grade students who are not familiar yet with systems of equations.
To see many other similar solved problems,  look into the lessons
    - More complicated coin problems
in this site.