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Question

# Two friends A and B simultaneously start running around a circular track . They run in the same direction. A travels at 6 m/s and B runs at b m/s. If they cross each other at exactly two points on the circular track and b is a natural number less than 30, how many values can b take

CAT 2021

A
B
C
D
##### 5
Solution

Correct option is

(A)

Let length of the track = x

time taken to meet for the first time

Time taken for a lap for A= x/6

Time taken for a lap for B= x/6

so , total time to meet for first time at starting point = LCM of (x/6 , x/b) which is equal to = x/HCF(6,b)

hence , number of meeting points on track = time taken to meet at starting for first meet = Relative speed / HCF of (6,b)

so , 6-b/HCF(6,b)=2 : or , b-6/HCF(6,b) = 2

the three different values which satisy the eqations are b = 2, 10 18