This answer gives the correct total value, but it is not the right number of coins. There should be 21 coins altogether, not 18. The correct equations with which to start the problem are:

(A) $\displaystyle{d}+{q}={21}\usepackage{cmbright}\Huge\$

(B) $\displaystyle{0.10}{d}+{0.25}{q}={2.70}\usepackage{cmbright}\Huge\$