Answer:
9
Step-by-step explanation:
Well, let's take a closer look at your problem. We know that Sergio can read 3 chapters in 1.5 hours. If he reads for 18 hours, then we can multiply 1.5 by 6, since 3 times 6 = 18. 1.5 times 6 is 9.
We can also set up a proportion and solve from there..
3/1.5 = 18/x