Monthly Basic Pay Calculation Base

How the monthly basic pay salary is calculated? Is it remains fixed irrespective of any other factor or it depends on the factors like no. of days employee is present?

E.G. if person has monthly CTC of Rs. 30000 and then his basic pay is suppose 30000*50% = Rs.15000 (Considering he is present for whole month) but 

if is only present of 23 days out of 30 days of month then how is his basic pay is calculated -

CTC (30000)*50%= 15000

OR

CTC ((30000*23)/30)=23000,so (23000*50%) = 11500

OR

Some other way,Please guide.