how much NaOH which is 25% pure required to neutralise 100mL 10N HCl

mols NaOH = M x L = 0.1 x 10 = 1 mol.

So you will need 1 mol of pure NaOH or 40 g. Since the NaOH you have is only 25% pure you will need 160 g of that material to obtain 40 g pure NaOH.