NaOH is a strong base and HCl is a strong acid. To neutralise one mole of acid , we will equal volume of one mole of base ( NaOH).
Using this fact we can write
number of moles of HCl react will be equal to the number of moles of NaOH.
number of moles of HCl = molarity of HCl x Volume of HCl ........ (a)
number of moles of NaOH = molarity of NaOH x Volume of NaOH ....(b)
using (a) and (b) we get,
molarity of HCl x Volume of HCl = molarity of NaOH x Volume of NaOH
40.0 mL x 0.20 M = 0.25 M x