A car travels the first one-third of a certain distance with a speed of 10 km/hr, the next one-third distance… · ABC26GN3668
Subject: General Aptitude · Chapter: Time and Distance
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 whole distance travelled be $x$ km and the average speed of the car for the whole journey be $y \mathrm{km} / \mathrm{hr}$. Then, $$\frac{(x / 3)}{10}+\frac{(x / 3)}{20}+\frac{(x / 3)}{60}=\frac{x}{y} \Leftrightarrow \frac{x}{30}+\frac{x}{60}+\frac{x}{180}=\frac{x}{y} \Leftrightarrow \frac{1}{18} y=1 \Leftrightarrow y=18 \mathrm{km} / \mathrm{hr} .$$