pH of acid or base mixtures

Exercise 1

Determine the $pH$ of the solution obtained by mixing $V_1$ = 100 $mL$ $HI$ 0.25 $M$ with $V_2$ = 500 $mL$ $HBr$ 0.55 $M$

$HI$ ,$ HBr$ :    strong acids $n_{HI}$ $=$ $c_{HI}\cdot V_1$ = 0.025 $mol$ $n_{HBr}$ $=$ $c_{HBr}\cdot V_2$ = 0.275 $mol$ $pH$ $=$ $-log\frac{n_{HI} + n_{HBr}}{V_1+V_2}$ = 0.301