Question

A certain comet is visible on earth once in every x years - that is, it has a periodicity of exactly x years. Three of the years in which it was seen are 1780, 1861 and 1987. Which of these could be the value of x?
a) 2 b) 5 c) 6 d) 9
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Answer 1

Answer:

9 is correct answer

as

1861-1780= 81

as it is multiple of 9 so answer is 9

Answer 2

Answer:

d) 9

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