In a 200 m race Arjun beats Vivek by 20 m, and in a 200 m race Vivek beats Kunal by 25 m. In a 200 m race between Arjun and Kunal, Arjun would beat Kunal by how many metres?

In the time Arjun runs 200 m, Vivek runs 180 m. Kunal covers 175 m for every 200 m of Vivek's, so while Vivek does 180 m Kunal does $180\times 175/200=157.5$ m. Arjun therefore finishes 200 − 157.5 = 42.5 m ahead of Kunal. (Adding the two margins to get 45 m forgets that Vivek's lead over Kunal is being measured over only 180 m, not the full 200 m.)