Calculate The Tax On Any Income

I'm trying to write a function that takes a single argument grossIncome of type Currency. It should calculate the tax on any income using the following tax schedule

1) If income is less than or equal to $15000 there is no tax.
2) If income is greater than $15000 and less than or equal to $75000 the tax is 15% of all income greater than $15000
3) If income is greater than $75000 the tax is 15% of all income between $15000 and $75000 plus 20% of all income greater than $75000

Then I want to write a sub that asks for the user's income and have the above function calculate the tax on the income and say the tax in a message box.