We have to calculate the salary for one day. Normally the number for days in a month is taken as 26. If you divide the salary by 26,the one day salary can be calculated. Then multiply by 30. For example salary/26*30.
In your case you have five days working. In such cases the number of working hours is extended beyond the normal eight hours working. But the weekly hours remains the same i.e. may be 48 hours equivalent to 6 days working. Therefore basis of calculation should be same.