If L, M, and S represent the large, medium, and small perimeters, then their total is $\displaystyle{L}+{M}+{S}={72}\usepackage{cmbright}\Huge\$ . The large is the sum of the small and medium: $\displaystyle{L}={M}+{S}\usepackage{cmbright}\Huge\$ , or $\displaystyle{L}-{M}-{S}={0}\usepackage{cmbright}\Huge\$ . The small is 20 inches less than the large: $\displaystyle{S}={L}-{20}\usepackage{cmbright}\Huge\$ , or $\displaystyle{L}-{S}={20}\usepackage{cmbright}\Huge\$ .