Task: Question Answering
Input (English): Problem: a , b , k start from the same place and travel in the same direction at speeds of 30 km / hr , 40 km / hr , 100 km / hr respectively . b starts two hours after a . if b and k overtake a at the same instant , how many hours after a did k start ? 
Options: a ) 4.2 , b ) 4.8 , c ) 5.6 , d ) 6.4 , e ) 7.2
Output (English): c