The ratio of the investments of A, B, and C is \( 6000: 7000: 8000 \). To simplify the ratio, divide each term by 1000:
\[
A : B : C = 6 : 7 : 8.
\]
Now, the total ratio is \( 6 + 7 + 8 = 21 \). The total profit is Rs.3150, so B's share is:
\[
B's \, \text{share} = \frac{7}{21} \times 3150 = 1050.
\]