A can do a work in 6 days. B can do the same work in 12 days. If both A & B are working together in how many days they will finish the work?  Select the correct option.
A) 3 B) 5 C) 4 D) 2 E) 6

Rationale: "a rate = 1 / 6 b rate = 1 / 12 ( a + b ) rate = ( 1 / 6 ) + ( 1 / 12 ) = 1 / 4 a & b finish the work in 4 days correct option is c"
Answer: C