Answer: 0.0400 M
Step-by-step explanation:

To calculate the volume of acid, we use the equation given by neutralization reaction:

where,
are the n-factor, molarity and volume of acid which is

are the n-factor, molarity and volume of base which is NaOH.
We are given:

Putting values in above equation, we get:

Thus the original concentration of the HCl solution is 0.0400 M