600mL of 0 . 01MHCl is mixed with 400mL of 0 . 01M H _ 2 SO _ 4 ....

The number of millimoles = $Molarity \times Volume$ in $mL$

$HCl$ produce one and ${H}_{2}{SO}_{4}$ produce two ${H}^{+}$ ions respectively.

Total milimoles of ${H}^{+} = \left(600 \times 0 . 01\right) + \left(400 \times 0 . 01 \times 2\right) = 14$
Thus, moles of hydrogen ion:
$⇒ \left[{H}^{+}\right] = \dfrac{14}{1000} = 14 \times {10}^{- 3}$
$⇒ pH = - log [ {H}^{+} ] = - log [ 14 \times {10}^{- 3} ]$

$⇒ pH = 3 - \log 14$

$= 1 . 86$

$= 186 \times {10}^{- 2}$