Question

A boy runs for 20min at uniform speed of 18km/h at what speed should he run for the next 40 min so that average speed comes 24 km/h

Answer 1

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→ Given :     A boy runs for 20 min at uniform speed of 18 km/h at what speed should he run for the next 40 min so that average speed comes 24 km/h

→ Answer :

       For the next 40 min he should run with 27 km/h

→ Explanation :

       $t_1=20 min , v_1=18 km/h,D_1=?\\\\t_2=40 min,v_2=?,D_2=?,A_{speed}=24km/h$

       $speed =\frac{distance}{time} \\\\=>v_1=\frac{D_1}{t_1} \\\\=>D_1=v_1.t_1\\\\=>D_1=18*20=360km$

       $v_2=\frac{D_2}{t_2}\\\\ =>D_2=v_2.t_2\\\\=>D_2=v_2*40\\\\=>D_2=40v_2$

        $Now,Average_{speed}=\frac{D_1+D_2}{t_1+t_2} \\\\=>24=\frac{360+40v_2}{20+40} \\\\=>1440=360+40v_2\\\\=>40v_2=1080\\\\=>v_2=27kn/h$

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