Two friends A and B run on a 550 m circular track such that A (who is faster) overtakes B in the middle of A's 6th round for the first time.If they were to run on a stright track of length 10000 m, find approximately how much headstart A can give to B such that they finish the race at the same time?
A 1818m B 1837m C 1762m D 1265m

[SOLUTION] Since A overtakes B in the middle of the 6th round,B must be in the middle of the 5th round. So A has covered 5.5 times the track length while B has covered 4.5 times the track length. For every 5.5m covered by A,B covers 4.5 m. 11m covered by A=9 m covered by B Headstart in 11m race=2m Headstart in 10000m race=(2*10000)/11=1818m
In the solution, it is said B must be in middle of the 5th round. How did we reach to this conclusion?