Subject: mathqa
Question: a , b , k start from the same place and travel in the same direction at speeds of 30 km / hr , 40 km / hr , 60 km / hr respectively . b starts 6 hours after a . if b and k overtake a at the same instant , how many hours after a did k start ?
Option: 
A: 4
B: d ) 12
C: e ) 5
D: 4.5
Answer: B