pH of acid or base mixtures

Exercise 1

Determine the $pH$ of the solution obtained by mixing $V_1$ = 50 $mL$ $HNO_3$ 0.2 $M$ with $V_2$ = 400 $mL$ $HBr$ 0.15 $M$

$HNO_3$ ,$ HBr$ :    strong acids $n_{HNO_3}$ $=$ $c_{HNO_3}\cdot V_1$ = 0.01 $mol$ $n_{HBr}$ $=$ $c_{HBr}\cdot V_2$ = 0.06 $mol$ $pH$ $=$ $-log\frac{n_{HNO_3} + n_{HBr}}{V_1+V_2}$ = 0.808