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If a third of the distance is covered at a speed of 20 mph and the balance at a speed of 60 mph, what is the average speed to cover the entire distance?

1. 40 mph
2. 34.66 mph
3. 36 mph
4. 33.33 mph
5. 30 mph

I used the formula 2ab/(a + b) and got 30 mph as the answer. The OA is different. Any ideas why?

The formulae will work if the distances covered are equal.

In this question, one-third and two thirds is the ratio in which the distances have got covered for the different speeds.

I solved it by assuming a total distance. I took 180 miles as the total distance.

So, one-third will be 60 miles and two-thirds will be 120 miles. Time taken to cover 60 miles @ 20 mph is 3 hours. Time taken to cover 120 miles @ 60 mph is 2 hours.

Average speed = Total distance / (total time) = 180 / 5 = 36 miles.

Is that the OA?