Avg or expected cycle time
= 40% * 79.1 + 60% * 84.2
= 82.16 ns
Now, 1 word read/written $\equiv$ 1 cycle
i.e 1 word read/written $\equiv$ 82.16 ns
$\therefore$ ? $\equiv$ 1 sec
Thus, answer
= $\frac{10^{9}}{82.16}$ mega-words/sec
= 12.17 megawords/sec