A can do a work in 4 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?  Choose one
A) 3
B) 5
C) 4
D) 2
E) 6

Rationale: a rate = 1 / 4 b rate = 1 / 16 ( a + b ) rate = ( 1 / 4 ) + ( 1 / 12 ) = 1 / 3 a & b finish the work in 3 days correct option is a
Answer: A