How many milliliters of 0.20 molar koh solution are needed to exactly neutralize 20. milliliters of 0.50 molar hcl?

The balanced equation for the neutralisation reaction between KOH and HCl is as follows
KOH + HCl --> KCl + H₂O
stoichiometry of KOH to HCl is 1:1
the number of HCl moles reacted = HCl concentration x volume 
number of HCl moles = 0.50 mol/L x 0.020 L = 0.01 mol
according to molar ratio of 1:1
number of KOH moles required to neutralise 0.01 mol of HCl = 0.01 mol of KOH
molarity of KOH = 0.20 M
there are 0.20 mol in 1 L Then 0.01 mol are - 0.01 mol / 0.20 mol/L = 50 mL 
50 mL of 0.20 M KOH required 

The answer to that would be magnesium

Abbreviated as mg-25.