Santa Delivers Presents

Geometry Level 4

Santa wants to deliver presents to every little boy and girl in the whole world. He plans his trip as follows: starting from the North Pole, he travels along a great circle cutting through both poles, ending up back on the North Pole. Next, he turns his path by 10 10 degrees, and travels the great circle, ending up back at the North Pole. He repeats this until he traveled through all the great circles 10 10 degrees apart. If he has to do all of this in 12 12 hours, then the smallest possible integer average speed in km/h for him to make it is N N . Find the last three digits of N N .

Assume that the Earth is a perfectly spherical 12 , 742 12,742 km diameter ball.


The answer is 46.

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3 solutions

We need to 1st find the Earth's Circumference in accord with the given assumptions. Approximating the value of pie as 22 / 7 22/7 does not work because of the Earth's Large Diameter. ( I know because I got two tries one with pie as 3.14 3.14 and one with pie as 22 / 7 22/7 both wrong.) So we use the more accurate value of the Earth's Circumference as π 12742 π * 12742 . The amount of great circles we have to traverse is given by 180 / 10 180/10 which is 18 18 . Hence our total distance is π 12742 18 π * 12742 * 18 . To get speed simply divide it by time giving us ( π 12742 18 ) / 12 (π * 12742*18)/12 . Just put these values in Wolfram Alpha to get the final answer which is minimum speed 60 , 045.2 \boxed{60,045.2} . Now enter last three digits after rounding up the value.

Wouldn't the last three digits be 45 because you have to round the 60,045.2 down?

Neelansh Bute - 7 years, 5 months ago

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Not quite, because if Santa went at 60,045 km/h, he wouldn't be able to deliver all of the presents in time!

The question asks for you to round the speed to the smallest possible value such that Santa is able to complete his mission on time.

Milly Choochoo - 7 years, 5 months ago

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Oh. I get it now, thanks!

Neelansh Bute - 7 years, 5 months ago

one trip around the world is R π R\pi . Since he is traveling 180 10 = 18 \frac{180}{10}=18 times around the world, because he doesn't want to travel the same path in the different direction. In total that is 18 R π 18R\pi . The journey has to finish in 12h, so v = 18 R π t = 18 × 12742 K m × π 12 h = 60045.26 v=\frac{18R\pi}{t}=\frac{18\times 12742Km\times \pi}{12h}=60045.26 . He mustn't travel slower, and since we are asked the smallest possible integer average that is 60 046 60\boxed{046}

Shamik Banerjee
Dec 29, 2013

Smallest possible integer average speed = Ceiling(Total Distance Traveled in km / Time taken in hours) = Ceiling(18 π 12742/10) = Ceiling(60045.2604) = 60046 km/h.

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