If 2 students from the same family attend in the same year (and neither has already attended for 2 years), the fee as otherwise computed is reduced by 1/9th.
Your son and daughter are both accepted to MathPath and let's say by our other fee policies you owe 2 x $4700 = $9400. You may take off (1/9)*9400 = 1044.44. Your final bill is $8355.56.
Now suppose you also pay 3 months early. Then you may take a further reduction of .015 x 8355.56 = 125.33, for a final fee of $8230.23. Or thought of another way, first you take 1.5% off 9400, obtaining 9259, and then you take 1/9th off that. The final fee is $8230.22. (It is a little theorem that these two reductions commute – or they would if we did not round off to the nearest penny until the end of the calculations.)
Your twin daughters are admitted and qualify for financial aid.
We tell you that the reduced tuition for each of them is $3150. Your bill at that point is $6300. Taking off 1/9th, your final
bill is $5600.