d=dimes, q=quarters
First equation: d+q=35
Second equation: 0.1d+0.25q=6.50
d+q=35 can be changed into the equation q=-d+35
Then, plug the first equation into the second equation. This is the method of substitution.
0.1d+0.25(-d+35)=6.50
0.1d-0.25d+8.75=6.50
-0.15d+8.75=6.50
-8.75 -8.75
-0.15d=-2.25
/-0.15 /-0.15
d=15
There are 15 dimes. You can check this answer by plugging in 15 as d in the first equation. Then you would get 20 quarters. These two numbers satisfy both equations.
