SOLUTION: A and B cycle along a circular track from the same point and at the same time, but in the opposite direction. It takes A 30 minutes to complete one round. How long does it take B t

Let L be the length of the circular track (in any length unit, for example, in meters))

and let x be the time for B to complete one round, in minutes (the unknown value under the problem's question).


Then the rate of moving for A is    meters per minute;

     the rate of moving for B is     meters per minute.


The total distance traveled by both cyclists in 21 minutes (till the meeting moment) is the track length L.


It gives you THIS total traveled distance equation


     {  = L.


Cancel L in both sides


     +  = 1.


It implies

 
     = 1 -  =  =  = 


Hence,


    x =  = 7*10 = 70.


ANSWER.  It requires 70 minutes for B to complete one round.