pH of acid or base mixtures

Exercise 1

Determine the $pH$ of the solution obtained by mixing $V_1$ = 200 $mL$ $HCl$ 0.5 $M$ with $V_2$ = 50 $mL$ $HBr$ 0.4 $M$

$HCl$ ,$ HBr$ :    strong acids $n_{HCl}$ $=$ $c_{HCl}\cdot V_1$ = 0.1 $mol$ $n_{HBr}$ $=$ $c_{HBr}\cdot V_2$ = 0.02 $mol$ $pH$ $=$ $-log\frac{n_{HCl} + n_{HBr}}{V_1+V_2}$ = 0.319