Motor-cyclist P started his journey at a speed of 30 km/hr. After 30 minutes, motor cyclist Q started from the same place but with a speed of 40 km/hr. How much time will Q take to overtake P?
[A] $1\ hours$
[B] $\frac{3}{8}\ hours$
[C] $\frac{3}{2}\ hours$
[D] $2\ hours$