If 30 oxen can plough 1/7th of a field in 2 days, how many days 18 oxen will take to do the remaining work?	Pick:
A) 30 days
B) 20 days
C) 15 days
D) 18 days
E) 21 days

Rationale: solution : we will use work equivalence method , 30 / 18 = ( 1 / 7 ) / ( 6 / 7 ) * x / 2 ; 5 / 3 = ( 1 / 6 ) * x / 2 ; or , x = 60 / 3 = 20 days . answer : option b
Answer: B