You found 6.60 on the ground at school, alpl in nickels, dimes, and quarters. you have twice as many quarters as dimes and 42 coins in all. how many of each type of coin do you have?

Let the number of nickel coins be n, the number of quarter coins be q and the number of dimes be d.

We know that 1 nickel= $0.05, 1 dime= $0.1 and 1 quarter= $0.25,

We found $6.60 means that : i) 0.05n+0.1d+0.25q=6.60

we have 42 coins in all mean: ii) n+d+q=42

we have twice as many quarters as dimes mean that iii) q=2d

using the last relation we can reduce the number of unknowns by writing all q's in terms of d's as follows:

i) 0.05n+0.1d+0.25q=6.60
ii) n+d+q=42

substitute q with 2d:

i) 0.05n+0.1d+0.25(2d)=6.60
ii) n+d+(2d)=42

i) 0.05n+0.1d+0.5d=6.60
ii) n+d+2d=42

i) 0.05n+0.6d=6.60
ii) n+3d=42

multiply the second equation by -0.2, to eliminate the d's:

i) 0.05n+0.6d=6.60
ii) -0.2n-0.6d=-8.4


i+ii)  

-0.2n+0.05n=6.60-8.4
-0.15n=-1.8
n=-1.8/(-0.15)=12

substitute in i):

0.05n+0.6d=6.60
0.05*12+0.6d=6.60
0.6+0.6d=6.60
0.6d=6.0
d=10

finally, from equation n+d+q=42:

12+10+q=42
22+q=42
q=42-22=20

# nickels=12, # dimes=10, # quarters=20