A, B and C enter into partnership. A invests some money at the beginning, B invests double the amount after 6 months, and C invests thrice the amount after 8 months. If the annual gain be Rs.18000. A's share is?  Pick:
A) rs . 7500
B) rs . 7200
C) rs . 6000
D) rs . 5750
E) rs . 6750

Rationale: "x * 12 : 2 x * 6 : 3 x * 4 1 : 1 : 1 1 / 3 * 18000 = 6000 answer : c"
Answer: C