Use a test point in each region. For example, point $\displaystyle{\left({1},{8}\right)}\usepackage{cmbright}\Huge\$ makes $\displaystyle{b}+{t}\le{8}\usepackage{cmbright}\Huge\$ false, so region 1 is not shaded. Point $\displaystyle{\left({3},{8}\right)}\usepackage{cmbright}\Huge\$ makes $\displaystyle{b}+{t}\le{8}\usepackage{cmbright}\Huge\$ false, so region 2 is not shaded. Point $\displaystyle{\left({6},{1}\right)}\usepackage{cmbright}\Huge\$ makes both inequalities true, so region 3 is shaded, while point (1, 1) makes $\displaystyle{35}{b}+{21}{t}>{210}\usepackage{cmbright}\Huge\$ false, so region 4 is not shaded.