question_answer

A car travels 30 km at a uniform speed of 40 km/h and the next 30 km at a uniform speed of 20 km/h. Find its average speed. (a) 35 km/h                         (b) 40 km/h (c) 22 km/h                         (d) 26.6 km/h

Answer:

\[{{V}_{ave}}=\frac{{{s}_{1}}+{{s}_{2}}}{\frac{{{s}_{1}}}{{{v}_{2}}}+\frac{{{s}_{2}}}{{{v}_{2}}}}=\frac{30km+30km}{\frac{30km}{40km/hr}+\frac{30km}{20km/hr}}\] \[=\frac{60km}{\left( \frac{3}{4}+\frac{3}{2} \right)hr}=\frac{60km}{\frac{18}{8}hr}\] \[=\frac{60\times 8}{18}km/hr=\frac{80}{3}=26.6km/h\]