q + d = 18
0.25q + 0.1d = 4.05
We will use substitution
Solving the first equation for q
q = 18 -d
Substituting this into the second equation
.25(18-d) + .1d = 4.05
Distribute
4.5 - .25d +.1d = 4.05
Combine like terms
4.5 - .15d = 4.05
Subtract 4.5 from each side
-.15d = -.45
Divide each side by -/15
-.15d/-.15 = -.45/-.15
d = 3
We have 3 dimes
Now we can find the number of quarters
q = 18-d
q = 18-3
q = 15
We have 15 quarters