SOLUTION-I: HA and HB are two weak acids, 1 litre 0.2M HA is mixed with 1 litre $0.2MHB$ . pH of the resultant solution is x (Given K_a for HA= $2\times {10}^{-5}$, K_a for HB= $7\times {10}^{-5}$ )
SOLUTION-II: 0.1M $H_{2}S$ solution in which enough HCl has been added to produce $\left[H^{+}\right]$  equal to $2\times {10}^{-4}$ M. In this solution $\left[S^{-2}\right]$ concentration is found to be $y\times {10}^{-15}M.$
(Given for $H_{2}S$: $K_1={10}^{-7},K_2={10}^{-14}$) $(\log 3=0.48)$