Let their investments be Rs. x for 12 months, Rs. y for 8 months and Rs. z for 6 months respectively.
Then, 12x : 8y : 6z = 4 : 6 : 8
Now, 12x/8y = 4/6 <=> 9x=4y <=> y=9x/4
And, 12x/6z = 4/8 <=> 4x=z <=> z=4x
Therefore, x : y: z = x : 9x/4: 4x = 4 : 9 : 16