Subject: General Aptitude · Chapter: Time and Distance · Exam: 2016 · Marks: · Difficulty:
A car travels the first one third of a certain distance with a speed of 10 km/hr, the next one third distance with a speed of 20 km/hr and the last one third distance with a speed of 60 km/hr. The average speed of the car for the whole journey is
(a)18 km/hr
(b)24 km/hr
(c)30 km/hr
(d)36 km/hr
Answer
Answer (as printed): A
Explanation
Let the distance travelled by a car be $x \mathrm{km}$. First $\frac{x}{3} \mathrm{km}$ distance cover at speed of 10 km/hr Second $\frac{x}{3} \mathrm{km}$ distance cover at speed $=20 \mathrm{km} / \mathrm{hr}$ Third $\frac{x}{3} \mathrm{km}$ distance cover at speed $=60 \mathrm{km} / \mathrm{hr}$ According to given information $$\begin{aligned} \text { Total time } & =\frac{x}{3 \times 10}+\frac{x}{3 \times 20}+\frac{x}{3 \times 60} \\ & =\frac{x}{30}+\frac{x}{60}+\frac{x}{180} \\ & =\frac{6 x+3 x+x}{180} \\ & =\frac{10 x}{180}=\frac{x}{18} \end{aligned}$$ Average speed $=\frac{x \times 18}{x} \mathrm{km} / \mathrm{hr} .=18 \mathrm{km} / \mathrm{hr}$