Question 1146173

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The starting point is to immediately subtract the 50 pennies to arrive at 200 cents worth of nickels and dimes.   

 N = number of nickels
 30-N = number of dimes

5N + 10(30-N) = 200
-5N + 300 = 200
   5N = 100
    N = 20   nickels  -->  and (30-20) = 10 dimes

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Check:  5*20 + 10*10 = 100 + 100 = 200 cents (that plus the original 50 pennies gets you to 250 cents in the coin bank).


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Some tutors will show you other ways to solve it.  Those are probably fine methods, I just prefer the mechanical method above.