Question 37847
Let N = # of nickels
Let D = # of dimes
Let Q = # of quarters
Thus we know
N + D + Q = 255
and for our value equation, we have
5N + 10D + 25Q = 4125 (no decimal since we're talking cents here)
plus we know that 
D = 2N
so substitute that in to both equations and get
N + 2N + Q = 255
5N + 20N + 25Q = 4125
and
3N + Q = 255
25N + 25Q = 4125
Now solve for Q in the first and substitute into the second...
Q = -3N + 255
25N + 25(-3N + 255) = 4125
25N - 75N + 6375 = 4125
-50N = -2250
N = 45
Thus D = 2N = 90
and Q = 255 - N - D = 120