20.0 mL of 5.0 M HCl solution contains
0.0200 L * 5.0 M = 0.10 mol
of HCl.
Mixing the HCl and NaOH solutions results in
HCl + NaOH ==> NaCl + H2O
which is to say 0.10 mol of HCl reacts with 0.10 mol of NaOH. If only 10.0 mL of NaOH solution is required, then its molarity x is such that
0.0100 L * x M = 0.10 mol => x = 10. M
