You may have chosen this answer because the number of dimes is twice the number of pennies and the number of coins is 18. However, to be correct the values need to make each of the conditions true. The correct system to start with is:

$\displaystyle{p}+{n}+{d}={18}\usepackage{cmbright}\Huge\$

$\displaystyle{0.01}{p}+{0.05}{n}+{0.10}{d}={1.08}\usepackage{cmbright}\Huge\$

$\displaystyle{2}{p}={d}\usepackage{cmbright}\Huge\$ or $\displaystyle{2}{p}-{d}={0}\usepackage{cmbright}\Huge\$