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Bunuel
Alfred bikes from home to a restaurant at a constant rate of 10 miles per hour. Reaching the restaurant, he realizes that he forgot a wallet at home and returns back, by the same route, at a constant rate of 12 miles per hour. After picking the wallet in negligible time, he bikes to the restaurant again by the same route at a constant rate of 15 miles per hour. What is the average rate of Alfred for the entire, three-leg trip?

A. 11 miles per hour
B. 12 miles per hour
C. 12.5 miles per hour
D. 13 miles per hour
E. 14 miles per hour

Formula: When distance travelled is same, average speed is Harmonic mean of all the speeds
—> \(\frac{n}{S_{n}} = \frac{1}{S_{1}}+ \frac{1}{S_{2}} + . . . . + \frac{1}{S_{n}}\)
Let the average speed = x
—> \(\frac{3}{x}= \frac{1}{10}+ \frac{1}{12}+ \frac{1}{15}\)
—> \(\frac{3}{x}= \frac{(6 + 5 + 4)}{60}\)
—> \(\frac{3}{x}= \frac{15}{60}= \frac{1}{4}\)
—> \(x = 12\)

IMO Option B

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This can be solved by using the simple formula :-

3abc/(ab+bc+ca); when one travels at a speed a for 1/3rd distance, at speed b for 1/3rd distance and at speed c for the rest 1/3rd distance.

Here, the same distance is being covered 3 times hence the formula can be used.

(3*10*12*15)/(10X12 + 12X15 + 15X10)

=12 miles per hour.

Hence, B is the answer.
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Bunuel
Alfred bikes from home to a restaurant at a constant rate of 10 miles per hour. Reaching the restaurant, he realizes that he forgot a wallet at home and returns back, by the same route, at a constant rate of 12 miles per hour. After picking the wallet in negligible time, he bikes to the restaurant again by the same route at a constant rate of 15 miles per hour. What is the average rate of Alfred for the entire, three-leg trip?

A. 11 miles per hour
B. 12 miles per hour
C. 12.5 miles per hour
D. 13 miles per hour
E. 14 miles per hour

We can use the formula: average = total distance/total time

average = 3d/(d/10 + d/12 + d/15)

Multiplying by 60/60, we have:

average = 180d/(6d + 5d + 4d)

average = 180d/15d = 12

Answer: B
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Bunuel
Alfred bikes from home to a restaurant at a constant rate of 10 miles per hour. Reaching the restaurant, he realizes that he forgot a wallet at home and returns back, by the same route, at a constant rate of 12 miles per hour. After picking the wallet in negligible time, he bikes to the restaurant again by the same route at a constant rate of 15 miles per hour. What is the average rate of Alfred for the entire, three-leg trip?

A. 11 miles per hour
B. 12 miles per hour
C. 12.5 miles per hour
D. 13 miles per hour
E. 14 miles per hour


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The Magic Wand: \(\frac{3XYZ}{XY+YZ+ZX}\)
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