Money spent
on party
Marginal social benefit
$0
(6 × $0.05) + (10 × $0.13) = $1.60
1
(6 × $0.04) + (10 × $0.11) = $1.34
2
(6 × $0.03) + (10 × $0.09) = $1.08
3
(6 × $0.02) + (10 × $0.07) = $0.82
4