Money spent
on party
Marginal social benefit
$0
(10 × $0.05) + (6 × $0.13) = $1.28
1
(10 × $0.04) + (6 × $0.11) = $1.06
2
(10 × $0.03) + (6 × $0.09) = $0.84
3
(10 × $0.02) + (6 × $0.07) = $0.62
4