Question

An automobile driver travel from a plane to hill station $120\text{km}$ away at an average speed of $30\text{km/hour}$. He then makes a return trip at an average speed of $25\text{km/hour}$. He then covers another $120\text{km}$ at an average apeed of $50\text{km/hour}$. His average speed (in$\text{km/hour}$) over the entire distance of $360\text{km}$ will be

• A. $35$
• B. $45$
• C. $\frac{225}{7}$
  
• D. $\frac{200}{7}$

Answer 1

The correct option is C $\frac{225}{7}$


Average speed $=\frac{\text{total distance}}{\text{total time}}$
$=\frac{120+120+120}{\frac{120}{30}+\frac{120}{25}+\frac{120}{50}}$

$=\frac{3}{\frac{1}{30}+\frac{1}{25}+\frac{1}{50}}\text{km/hour}=\frac{225}{7}\text{km/hour}$