Let $\displaystyle{d}=\usepackage{cmbright}\Huge\$ the number of dimes and $\displaystyle{q}=\usepackage{cmbright}\Huge\$ the number of quarters. Dimes are worth $0.10 and quarters are worth $0.25, so:

(A) $\displaystyle{d}+{q}={21}\usepackage{cmbright}\Huge\$ (B) $\displaystyle{0.10}{d}+{0.25}{q}={2.70}\usepackage{cmbright}\Huge\$

Solve (A) for d: $\displaystyle{d}={21}-{q}\usepackage{cmbright}\Huge\$

Substitute into (B): $\displaystyle{0.10}{\left({21}-{q}\right)}+{0.25}{q}={2.70}\usepackage{cmbright}\Huge\$

Multiply, and combine like terms: $\displaystyle{2.10}+{0.15}{q}={2.70}\usepackage{cmbright}\Huge\$

The solution of this equation is $\displaystyle{q}={4}\usepackage{cmbright}\Huge\$ , which gives you $\displaystyle{d}={21}-{4}={17}\usepackage{cmbright}\Huge\$ .