Puzzle 107 - solution

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The question was 

Two missiles speed directly toward each other, one at 9,000 miles
per hour and the other at 21,000 miles per hour. They start 1,317 miles apart. Without using pencil and paper, calculate how far apart they are one minute before they collide.

 If two objects are moving in opposite direction in speeds x and y , their relative speeds are x+y. (If they are moving in the same direction, their relative speeds are x-y).

In this question, they are moving towards each other at a speed of 21000+9000 = 30000 miles/hour = 30000/60 = 500 miles/minute.

Which means, just one minute before their collision, they were 500 miles apart.

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