Butanoic Acid

Find the pH during the titration of 20.00mL of 0.1000 M butanoic acid, CH3CH2CH2COOH (ka= 1.54x10^-5) with .1000 M NaOH solution after the following additions of titrant
a) 12.00 mL 
b)20.60 mL
c) 29.00 mL 

I got the answer to a and it was 4.98, however, I can't seem to get the other two and I have done several different methods