pH of acid or base mixtures

Exercise 1

Determine the $pH$ of the solution obtained by mixing $V_1$ = 300 $mL$ $HBr$ 0.25 $M$ with $V_2$ = 200 $mL$ $HClO_4$ 0.4 $M$

$HBr$ ,$ HClO_4$ :    strong acids $n_{HBr}$ $=$ $c_{HBr}\cdot V_1$ = 0.075 $mol$ $n_{HClO_4}$ $=$ $c_{HClO_4}\cdot V_2$ = 0.08 $mol$ $pH$ $=$ $-log\frac{n_{HBr} + n_{HClO_4}}{V_1+V_2}$ = 0.509