If 20 milliliters of 4.0 m naoh is exactly neutralized by 20 milliliters of hcl, the molarity of the hcl is

Volume of NaOH = 20 mL / 1000 = 0.02 L

Molarity NaOH = 4.0 M

number of moles NaOH :

n = M * V

n = 4.0 * 0.02

n = 0.08 moles of NaOH:

Finally we calculate the number of moles of HCl in the solution from the stoichiometry of the reaction :
HCl + NaOH = NaCl + H₂O

1 mole HCl 1 mole NaOH
? moles HCl  0.08 moles NaOH

moles HCl = 0.08 * 1 / 1

= 0.08 moles of HCl

Volume of HCl = 20 mL / 1000 = 0.02 L

M ( HCl ) = n  / V

M = 0.08 / 0.02

= 4.0 M

hope this helps!