340 pages; 20% decrease 340 pages; 20% decrease

272 pages

Step-by-step explanation:

First, think of 100% of "x" as x*1.00

Thus, 80% of "x" is x*0.8

So, a 20% decrease of "x" would be x * (1.00-0.2) = x*0.8

We know x is 340 pages, so a 20% decrease would be 340*0.8=272 pages.