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 , 120 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 ) 3 , b ) 4 , c ) 6 , d ) 8 , e ) 10
Output (English): c