A runs twice as fast as B and gives B a start of 42m.How long should the racecourse be so that A and B might reach in the same time?
Choose the correct choice from the following:  A) 75 m .  B) 80 m .  C) 84 m .  D) 100 m .  E) none of the above

Rationale: "ratio of speeds of a and b is 2 : 1 b is 42 m away from a but we know that a covers 1 meter ( 2 - 1 ) more in every second than b the time taken for a to cover 42 m is 42 / 1 = 42 m so the total time taken by a and b to reach = 2 * 42 = 84 m answer : c"
Answer: C