Ajay walks at a speed of 4 km/hr. He doubles his speed after reaching exactly half way. He walks for 12 hours in all. What is the total distance travelled by him?

64 km

Assume total distance travelled by Ajay = D

Ajay's speed = 4 km/hr

Reaches  a distance of D/2

Time taken T1 = (D/2)/4 = D/8

Ajay doubles his speed = 8 km/hr

now he travels another distance = D/2

Time taken T2 = (D/2)/8 = D/16

Given total time = 12 hours

T1 + T2 = D/8 + D/16 = 12

3D/16 = 12

D = 64

The correct answer is Option (3) → 64 km