Adam and Harry are travelling from city X to city Y in their respective cars. Adam reaches city Y in 3 hours whereas Harry reaches 45 minutes after Adam. If the speed of Harry’s car is less than Adam’s car by 12 km/hr, find the speed of Adam’s car.

Distance b/w X and Y = Z km
Adam takes time = 3 hrs
Harry takes time = 45/60 = 0.75 hrs
Speed of Adam's Car = m
Speed of Harry's Car = m - 12
Speed = Distance/time or Distance X time
For Adam's Car,
M = Z/3
3m = Z
For Harry's Car,
M_12 = Z/0.75
M_12 = 4M
m - 12 = 4m
3m = -12
m = -4
But speed cam't be negative
m = 4 km/h
Speed of Adam's Car = m = 4 km/h

2. root 6 is the right answer