A 25.0 mL sample of a 0.0600 M solution of aqueous trimethylamine is titrated with a 0.0750 M solution of HCl. Calculate the pH of the solution after 10.0, 20.0, and 30.0 mL of acid have been added; pKb of (CH3)3N = 4.19 at 25°C.

I already calculated pH of the solution after 10.0 and 30.0 mL were added, but I am having trouble finding the pH after 20.0 mL is added.Please help me, thank you.