NH3 is neutralised by the equation:
HCL + NH3 -> NH4CL
In this equation there is a one to one relationship in terms of the number of moles of each reactant. I.e. To neutralise 1 mole of NH3 we require 1 mole of HCL.
To calculate the concentration of NH3 required, we must first calculate the number of moles of HCL used.
volume HCL = 35.5mL = 0.0355 litres
concentration HCL = 0.23M = 0.23 mole/litre
Note that the term "M" for concentration simply means moles/litre
number moles = concentration x volume
number moles HCL = 0.0355 x 0.23 = 0.008165 moles HCL
based on the equation, we know the number of moles of NH3 must be the same
moles NH3, n = 0.008165
volume NH3, v = 20.0mL = 0.020 litres
n = c x v
c = n / v
c = 0.008165 / 0.020
i.e. the concentration of NH3 would be 0.41 moles/litre or 0.41M
This intuitively makes sense because there is less volume of NH3 required to be neturalised, in a one-to-one mole relationship. So the concentration of NH3 would need to be higher than that of HCL.