andy walks at a certain speed. With that speed , he covers a distance of 4 km. he then doubles his speed and covers another 16km.The total time taken by him is 3 hours What was his speed???

let andy's speed be x km/h

time taken with this speed to cover 4 km/h = 4/x h

time taken to cover 16 km with a speed of 2x = 16/2x h

total time taken is 4/x +16/2x

4/x +16/2x = 3

8 + 16/2x =3

24 = 6x

x = 4

thus andy's speed is 4 km/ h