I've calculated and my answer is 1.66 mL.

But I'm not sure bout the ans plus my working is too complicated, can you show me how u get the ans?

Correct me if I'm wrong.

Thanks!

my version:

1) to change % to concentration

For H2SO4: 4.74*10/MW

_{H2SO4} = 0.48 mol L

^{-1} For NaOH: 48.5*10/MW

_{NaOH} = 12.13 mol L

^{-1}2) 0.48mol of 21mL H2SO4 is eq to 0.01008 mol

3) since acid to alkali ratio is 1:2, therefore moles of NaOH needed is 0.02016 mol

4) therefore volume needed for 48.5% NaOH is

0.02016*1000/12.13 = 1.66 mL

Thanks again!