Number of quarters, dimes and nickles. I'm solving a problem which states the following;

Mary has $3.00 in nickels, dimes, and quarters. If she has twice as
  many dimes as quarters and five more nickels than dimes, how many
  coins of each type does she have?

I transcribed the relationships as such;
$2q=d, n=d+5$
And i need to solve the following;
$
3 = 0.01n+0.1d+0.25q \therefore\\
3=0.01(d+5)+0.1(2q)+0.25q \therefore\\
3=0.01d+0.45q+0.05 \therefore\\
3=0.01(2q)+0.45q+0.05\therefore\\
3=0.47q+0.05\therefore\\
2.95 = 0.47q \therefore\\
6.4 = q
$
I've checked the answer online, but im interested and why this method failed. I'm not aware of rules i broke here.

One nickel is $5$ cents.
The equation is 
$$3=0.0\color{red}5n+0.1d+0.25q$$
$
3 = 0.05n+0.1d+0.25q \therefore\\
3=0.05(d+5)+0.1(2q)+0.25q \therefore\\
3=0.05d+0.45q+0.25 \therefore\\
3=0.05(2q)+0.45q+0.25\therefore\\
3=0.55q+0.25\therefore\\
2.75 = 0.55q \therefore\\
5= q
$
