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 walking 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 much distance is covered by Z on foot?
- $25$ km
- $10$ km
- $20$ km
- $15$ km