M, N and O started a business investing Rs. 4000, Rs. 6000 and Rs. 10000 respectively. After some months N left the business and after another 3 months O also left the business. If after 12 months their profit ratio is 8 ∶ 3 ∶ 10, after how many months did N leave?
1
2
2
5
3
3
4
7