Buffer Calculation

I am struggling with this buffer calculation:
The value of Ka for methanoic acid is 1.78 × 10– 4mol dm–3 at 25oC.
A buffer solution is prepared containing 2.35 × 10–2mol of methanoic acid and
1.84 × 10–2mol of sodium methanoate in 1.00 dm3 of solution.
2 (c) (i) Calculate the pH of this buffer solution at 25oC.

A 5.00 cm3 sample of 0.100 mol dm–3 hydrochloric acid is added to the buffer solution
in part (c) (i).
Calculate the pH of the buffer solution after this addition.

I know how to work out the first one, but the second confuses me. I thought after you work out the remaining X- and new HA you have to work out concentrations? The answer is 3.62 and I get that if I just put moles into the [H+] = Ka x [HX]/[X-]. Its from the AQA Chem4 Jan2013 paper if you want the mark scheme

You never have to work out concentrations with buffers as the two components of the buffer are by definition in the same solution. You can always work with moles.