0.1M solution of HCl is added to 0.180M solution of NaOH. (each 25ml) Concentration of OH-(aq) remaining in the solution, is 0.04M. explain...

Start by finding the number of moles of each reactant you
are using.

For HCl you have 0.0025
moles

For NaOH you have 0.0045
moles.

In the neutralization reaction all of the HCl will
react with 0.0025 moles of NaOH, leaving 0.0020 moles of NaOH.  But that remaining NaOH
is now in 50 mL of solution, not the original 25 ml you started
with.

0.002 moles of NaOH in 50 ml is equivalent to a 0.04
M solution of NaOH.