Here's a simple outline of a way you could do it:
echo '<form method="get" action="calculate.php">
Distance in Miles: <input type="text" name="distance" /><br />
Number of Stops: <input type="text" name="stops" /><br />
<input type="submit" name="speed" value="30 mph" /><br />
<input type="submit" name="speed" value="60 mph" /><br />
</form>';
calculate.php:
$travel_time = $_GET['distance'] / $_GET['speed'];
$travel_time += $_GET['stops'] * (5 / 60);
echo $travel_time . ' hours';