In future please attempt to answer at least, or post your specific difficulty.
Amanda walks eastwards at 4 m/sec
The dog walks east with respect to Amanda at 1 /sec.
(Thus the vel of the dog is 5 m/sec east. )
Hence the leash will unravel at 1 m/sec till it reaches its limit of 20 m.
At a vel of 1 m/sec, this will take 20 sec.
The speed of the dog is 5 m /sec with respect to the ground. Thus in 20 secs, it will cover
20 x 5 = 100 m.
