How far will the Betaville swallow travel towards Alphaville?
One snail starts from Alphaville at a speed of 1 km/h to get to Betaville, 21 kilometers away. At the same time, a swallow departs from Betaville towards Alphaville, at a speed of 30 km/h. As soon as the swallow reaches the snail’s position, it turns around. Arrived at Betaville, the swallow makes a U-turn again in the direction of the snail and so on. Knowing that the path taken by the two animals is the straight line between the two cities, how far will the swallow have traveled when the snail will reach Betaville?
Response
630 kilometers. The speed of the snail is 1 km/h and therefore implies that he covers the 21 kilometers in 21 hours. During this time, the swallow travels 30 x 21 = 630 kilometres.
The enigma, in a completely different version…
Martin Gardner, the great creator of mathematical games, proposed this enigma in a “cold war” version.
Of them missile move towards each other, the first at a speed of 9,000 km/h and the second at a speed of 21,000 km/h. Initially, they are 7,821 kilometers apart. How far apart will they be one minute before they collide?
Response
500 kilometers. The missile are moving towards each other at a speed of 30,000 km/h. One minute before the collision, they are therefore 30,000/60 = 500 kilometers apart.
Learn more about Hervé Lehning
Normalien and agrégation in mathematics, Hervé Lehning taught his discipline for a good forty years. Crazy about cryptography, member of the Association of encryption and information security reservists, he has in particular pierced the secrets of Henri II’s cipher box.
- His blog MATH’WORLD on Futura
- the latest book by Hervé Lehning :
Also to discover: The universe of secret codes from Antiquity to the Internet published in 2012 by Ixelles.
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