The mod operator is effectively part of the division operator: divide gives you the quotient, mod gives you the remainder.
In the example you give, 16 days at the fortnightly rate:
16 / 14 = 1 (since there are 14 days is a fortnight)
16 Mod 14 = 2 (since there is a remainder of 2)
So all you need to say is:
Dim daysAtFortnightRate = daysRented / 14
Dim daysAtDailyRate = daysRented Mod 14
Then all you have to do to work out the cost, is multiply each by the relevant rate and add then two values together.