Can someone show me the correct forumla to provide a net mothly figure

e.g
if a gross yearly wage figure is entered into A1 how can i show the tax paid
in A2 and the resulting nett monthly figure in A3

The formula has to work within the following thresholds:
0-$17,500 taxed at 12.5%
$17,501 - $40,000 taxed at 21%
$40,001 - $75,000.00 taxed at 33%
$75,001 and above taxed at 39%