Answer:
Customer Cost Marginal cost Total Revenue Profit
1 $30 0 10 -20
2 $32 2 20 -12
3 $35 3 30 -5
4 $38 3 40 2
5 $42 4 50 8
6 $48 6 60 12
7 $57 9 70 13
8 $68 11 80 12
Therefore, profit from the 7th customer load is the highest. So, he want 7th customer load.