What volume of .100 M NaOH will react completely with 21.00 mL of .500 M HCl?

Vb*ConcBase= Vacid*ConcAcid

Solve for Vb.

21.00 mL x .500 M HCl = 1.05e1
1.05e1 / .100 M NaOH = 1.05e2

right.