Kathleen can paint a room in 3 hours, and Anthony can paint an identical room in 4 hours. How many hours would it take Kathleen and Anthony to paint both rooms if they work together at their respective rates? Select
A) 8 / 15
B) 4 / 3
C) 15 / 8
D) 24 / 7
E) 15 / 4

Rationale: "( 1 / 3 + 1 / 4 ) t = 2 t = 24 / 7 answer : d"
Answer: D