HCl + NaOH ===> NaCl + H2O neutralization reaction
Note that the mole ratio is 1:1 meaning that 1 mole HCl reacts with 1 mole NaOH to produce 1 mole NaCl and 1 mole H2O.
Find moles of NaOH used:
18.45 ml x 1 L/1000 ml x 0.085 mol/L = 0.001568 moles NaOH used
This meant that there must be 0.001568 moles of HCl present in the 25.0 mls.
We want to find the molarity (M) which is moles HCl/liter of solution. We know the moles, and we also know the liters of solution (25.0 mls x 1 L / 1000 mls = 0.025 liters)
M = 0.001568 moles HCl / 0.025 liters = 0.06273 M = 0.0627 M (to 3 significant figures)