This answer gives the correct number of coins, but it is not the right total value. Fifteen dimes are worth $1.50, and 6 quarters are worth $1.50. This would be a total of $3.00, not $2.70. 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\$