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خطا

$s=\frac{\ell }{\Delta t}\Rightarrow \Delta t=\frac{\ell }{s}$ $\left. \begin{matrix} \Delta {{t}_{1}}=\frac{\ell }{60}+\frac{\ell }{20}=\frac{4}{60}\ell   \\ \Delta {{t}_{2}}=\frac{\ell }{40}+\frac{\ell }{40}=\frac{2}{40}\ell   \\ \end{matrix} \right\}\Rightarrow \Delta {{t}_{1}}\gt \Delta {{t}_{2}}$ $\Delta {{t}_{1}}-\Delta {{t}_{2}}=2/5\Rightarrow \frac{4}{60}\ell -\frac{2}{40}\ell =2/5\Rightarrow \ell =150km$