Question

From the same place at 7am A started walking in the north at the speed of 4km/hr.After 1hr B started cycling in the east at a speed of 8km/hr .At what time they will be at a dist of 20km apart from each other

Answer 1

According to question

speed of A =  u =  4 km

Thus after 1 hr,
                        distance traveled by A =  x = u * t = 4 Km

According to question

                     B - A = 20
                     B  - x = 20
                     B  =  20 + 4
                      B = 24 Km
Thus    t = 24 / 8 = 3 Hr

Thus at 3 hrs they will be that much apart          

Answer 2

Let the time taken for the distance between them to be 20km be t

Distance traveled by A = 4t
Distance traveled by B = 8t

By pythagoras theorem:

(4t)² + (8t)² = (20)²
16t² + 64t² = 400
80t² = 400
t²    = 5
t = √5 = 2.236 Hrs =  2hr 14 minutes

The time will be 7 a.m. + 2hr 14 min =  9.14 a.m.