If Abhi travels a certain distance at 6 km/h. he reaches his destination 12 minutes early, but if he travels at 4 km/h, he reaches his destination 10 minutes late. The speed (in km/h) at which he should travel to reach his destination on time is ........

Ratio of the speed = 6 : 4 = 3 : 2

Ratio of the time = 2 : 3

(when distance is constant then ratio of time is inverse to the ratio of the speed)

Time difference = 12 + 10 = 22 min

($$\because$$ he reaches 12 min early and 10 min late.) 

Ratio difference = 3 - 2 = 1

1 unit = 22 min

2 unit = 22 $$\times$$ 2 = 44 min 

When Abhi travels at  speed of 6km/hr he reaches his office in 44 min which is 12 min early so on time = 44 + 12 = 56 min

Distance = time \times speed = $$44 \times 6  = 264$$

speed to reach on time = 264/56  = 33/7 = $$4\frac{5}{7}$$