[tex]s=\frac{d}{t},\ where\ s-speed,\ d-distance,\ t-time\\\\s_1=\frac{1.5}{28}=\frac{15}{280}=\frac{3}{56}\\\\s_2=\frac{1.2}{10}=\frac{12}{100}=\frac{3}{25}\\\\s_a=\frac{\frac{3}{56}+\frac{3}{25}}{2}=\frac{\frac{75}{1400}+\frac{168}{1400}}{2}=\frac{243}{1400}\cdot\frac{1}{2}=\frac{243}{2800}\frac{miles}{minute}[/tex]
