${{d}_{1}}=40{{t}_{1}},{{t}_{2}}=\frac{1}{2}{{t}_{1}}\xrightarrow{{{t}_{1}}=\frac{{{d}_{1}}}{40}}{{t}_{2}}=\frac{{{d}_{1}}}{80}$
${{d}_{2}}={{v}_{av,2}}\times {{t}_{2}}=20\times \frac{{{d}_{1}}}{80}=\frac{{{d}_{1}}}{4}$
$\left| \Delta x \right|={{d}_{1}}-\frac{{{d}_{1}}}{4}=\frac{3{{d}_{1}}}{4}$
$\ell ={{d}_{1}}+{{d}_{2}}={{d}_{1}}+\frac{{{d}_{1}}}{4}=\frac{5{{d}_{1}}}{4}$
$\frac{{{s}_{av}}}{\left| {{v}_{av}} \right|}=\frac{\frac{\ell }{{{t}_{1}}+{{t}_{2}}}}{\frac{\left| \Delta x \right|}{{{t}_{1}}+{{t}_{2}}}}=\frac{\ell }{\left| \Delta x \right|}\xrightarrow[\left| \Delta x \right|=\frac{3{{d}_{1}}}{4}]{\ell =\frac{5{{d}_{1}}}{4}}\frac{{{s}_{av}}}{\left| {{v}_{av}} \right|}=\frac{5}{3}$