Ok let me explain the procedure to you:
The buffer solution is like this:
pH = pKa + log [salt]/[acid]
4 = 4.20 + log [salt] / [acid]
-0.20 = log [salt]/[acid]
If you Solve for [salt]/[acid]
This gives ratio of moles.
Take amount of acid be x L and then amount of salt will be 0.1-x
Now molarity*volume=number of moles
I know that with this you will be able to get to the result. Hope this helps
