Question

I’m trying to make a small calculator, as a personal project. I have a little example here, what I want:

The user(me) get paid 10$ from a customer. What I do, is first to subtract the percentage which is 2.75% so after the subtraction I will have 9.725$ back. Then I take 1.5$ in fee. So I will be down on 8.225$ in the end.

That one works as it should.

What I then want to calculate is.

If I want 10$ AFTER fees are paid, (the 2.75% and 1.5$), how much should the customer then pay? and how could I do the calculation, so I could use any amount of moneys I want to receive.

Hope any can solve this ‘easy’ math problem.

A little bonus I would like to do also, is, if I edit the percentage to 3% etc, it should work too. (If impossible, then never mind).

I’m feeling really lost here at the moment!

Here are what I currently do, but it just adds 2.75% and the 1.5$ to the amount I specified.

function retPercent($amount, $percent)
{
    return $amount + ($amount / 100 * $percent);
}

The problem here, is that I still don’t know how to calculate the percentage I actually should use (based on the 2.75%) that I get from the substraction.

Thank you a lot guys and girls!

Answer 1

$result = ($value + 1.5) / 0.9725;

Or more generally:

function myFunc($value, $percentage, $fee) {
    return ($value + $fee) / ((100-$percentage) / 100);
}

Other way round:

function myFuncTwo($result, $percentage, $fee) {
    return (((100-$percentage) / 100) * $result) - $fee;
}

Example Use:

echo myFunc('8.225', '2.75', '1.5'); //10
echo myFuncTwo('10', '2.75', '1.5'); //8.225