Let us consider that we have x instructions:
Time taken is 1ns for each x instruction so total =xns.
10% are sequential,
let us consider each instruction takes 1ns then,
(0.1x) instructions each takes 1ns so->(0.1x)ns
and, remaining all the instructions can be executed parallely in 1ns
So, total time =(0.1x+1)ns
Speed up=x/(0.1x+1) =10.