If it takes 54ml of 0.1 m naoh to neutralize 125 ml of a hcl solution with unknown concentration, what is the concentration of the hcl?

The balanced equation for the reaction between NaOH and HCl is as follows
NaOH + HCl ---> NaCl + H₂O
stoichiometry of NaOH to HCl is 1:1
number of NaOH moles reacted - 0.1 mol/L x 0.054 L = 0.0054 mol
number of HCl moles reacted = number of NaOH moles reacted 
since the molar ratio of acid to base is 1:1
therefore number of HCl moles reacted - 0.0054 mol
the number of moles of HCl in 125 mL - 0.0054 mol
therefore number of HCl moles in 1000 mL - 0.0054 mol / 125 mL x 1000 mL 
number of moles of solute in 1000 mL is known as the molarity concentration 
concentration of HCl is 0.0432 M

The answer would be solvent