Okay working on a timesheet--still--

I need to be able to take the total number of miles driven and subtract a
fixed number based on the total number of miles driven.

For 1-99 miles subtract 50 miles--so for 91 miles driven I need it to show
as 41 miles
For 100-199 subtract 25 miles--so for 175 miles driven I need it to show as
150 miles

Over 200 Subtract 0 miles-- so 225 as 225

Thanks!
--Ms Chewie