A british Concorde (BC) and a French Concorde (FC) flew in opposite directions around the world, a distance of 40 000 km. The BC covered half of its flight distance at a supersonice speed of 2500 km/h and the other half at a subsonic speed of 1000 km/h. The FC spent half of its flight time at 2500 km/h and the other half at 1000 km/h. Which Concorde completed the trip first, and by how many hours did it beat the other? (SIN'76)

Answer: FC by 5.1 h

The directions which they took are important for this question. Because the Earth rotates and there is the difference in time across all the world which we call in french 'decalage horaire'.