Answer:
Marginal Social Benefit = The Sum of all individual Benefit.
The Marginal social benefit of the first plant = Sum of first plant benefit of Robin, Tyler, and Ray
1. The marginal social benefit of the first = $47 + $41 + $31 = $119
2. The marginal social benefit of the second = $37 + $28 + $19 = $84
3. The marginal social benefit of the third = $13 + $6 + $2 = $21