Question : Ram travels with speeds of 15 km/hr, 10km/hr, and 12 km/hr for three equal distances. What is his average speed if the total distance travelled by him is 180 km?
Option 1: 15 km/hr
Option 2: 18 km/hr
Option 3: 20 km/hr
Option 4: 12 km/hr
Correct Answer: 12 km/hr
Solution :
Given: Ram travels with speeds of 15 km/hr, 10km/hr and 12 km/hr for three equal distances. The total distance travelled by him is 180 km.
So, he travels first $\frac{180}{3}$ = 60 km with 15 km/hr, next 60 km with 10 km/hr and the last 60 km with 12 km/hr speed.
The time taken for the first 60 km = $\frac{60}{15}$ = 4 hours
The time taken for the next 60 km = $\frac{60}{10}$ = 6 hours
The time taken for the last 60 km = $\frac{60}{12}$ = 5 hours
So, the average speed = $\frac{\text{Total distance}}{\text{Total time}}$
= $\frac{180}{4+6+5}$
= $\frac{180}{15}$
= 12 km/hr
Hence, the correct answer is 12 km/hr.
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