I'll assume I have this right:
Charge: customer has an account with the cab company. The customer doesn't hand over any money in the cab. The driver just calls in the net amount to the cab company, and they bill the customer (typically a company) at the end of the week/month.
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Maybe a different way of looking at things might help:
In your example, the money that you collected yourself is:
3.90
5.10
8.90
2.10
====
20.00
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Then you have a single "charge" item, that you haven't seen any of the cash for:
16.10
So, think of them separately, first.
Of the $20.00 dollars that you have, you need to hand over 60% of that to the company. I.e. that's $12.00 you owe them.
With the charge items (all one of it), they owe you your 40%. Hence they should give you $6.44 of what they have collected.
On balance, you owe them $12.00, less the $6.44 they owe you, so you send them $5.56 to get it straight.
Maybe occasionally you will have a day with a whole load of charge calls. On a day like that, the 40% cut of those calls (that the company owes you) may turn out to be more than the 60% from ordinary calls (that you owe them), so they'll pay you, for a pleasant change.
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The way you've done the calculation is just a different way of looking at it. You have lumped everything together, and done the 60% calculation. You've then calculated the sum of the charge calls (will is what the company already have "taken from you"), and paid them the difference. 100%-60%=40%.
All you have to be wary of is that you remember NOT to pay them when the answer comes out negative!
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"Striking up conversations with strangers is an autistic person's version of extreme sports."
Kamran Nazeer