To find the profit or loss, we need to calculate the total cost price (CP) and the total selling price (SP) of the toffees.

The cost price of each toffee is the same as the selling price, as the shopkeeper makes a 20% profit. So, the cost price per toffee is 100/120 = 5/6 of the selling price.

Let’s calculate the CP and SP for each customer:

Customer 1: Bought 10 toffees CP = 10 * (5/6) = 50/6 SP = 10 * 1 = 10

Customer 2: Bought 15 toffees CP = 15 * (5/6) = 75/6 SP = 15 * 1 = 15

Customer 3: Bought 20 toffees (forgot to pay) CP = 20 * (5/6) = 100/6 SP = 20 * 1 = 20

Customer 4: Bought 25 toffees CP = 25 * (5/6) = 125/6 SP = 25 * 1 = 25

Customer 5: Bought 30 toffees CP = 30 * (5/6) = 150/6 SP = 30 * 1 = 30

Now, let’s calculate the total CP and SP:

Total CP = (50/6) + (75/6) + (100/6) + (125/6) + (150/6) = 500/6 Total SP = 10 + 15 + 20 + 25 + 30 = 100

If the person who forgot to pay had paid his amount, the total SP would have been 100 + 20 = 120.

Profit = Total SP - Total CP Profit = 120 - (500/6) Profit = (720 - 500)/6 Profit = 220/6 Profit = 110/3

Therefore, the shopkeeper’s profit from these five customers, if the person who forgot to pay had paid his amount, would be 110/3 or approximately 36.67.