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X, Y and Z wish to go from place A to place B, which are separated by a distance of $70$ km. All the three persons start off together form A, with X and Y going by Luna at a speed of $20$ kmph. X drops V somewhere along the way and returns to pick up Z who has already started walkig towards B at a speed of 5 kmph. V. after being dropped by X, starts walking towards B at a speed of $5$ kmph. In this manner, all three of them reach B at the same time.

How long does it take them to go from A to B?

- $6.0$ hours
- $6.5$ hours
- $7.0$ hours
- $14.0$ hours