EMAT = p*(pfst) + (1 - p)*memory access time
where pfst = page fault service time
and p = page fault rate
Coming to your question,
EMAT = $\frac{1}{10^{6}}$*(10 ms) + (1 - $\frac{1}{10^{6}}$)*20 ns
= 10 ns+ 20 ns ( $\because$ $\frac{1}{10^{6}}$ is negligible as compared to 1 )
= 30 ns