Question

What volume (in ml) of $6$N must be added to $25$ ml of $2$N $HCl$ to get $100$ mL of $5$N solution?

Answer 1

As we know,
$N_1{V}_1+N_2{V}_2=NV$
$6\times x+2\times 25=5\times 100$
$⟹x=75$ mL

Hence, the volume that must be added is $75$ mL.