"you called number of pages he read on second day = P That's OK. But According to your formula "P + .20P = 60 If I define it so than (no. of pages on second day) + (20% of no. of pages on second day) = 60 Which is wrong!!!!! because as in question no. of pages on second day are 20% less than 1st day." This Y0UR error. the original post said "This was 20% more than he read on the second day" That is NOT the same as saying "what he read on the second day was 20% less than he read on the first day.
For example if he read 120 pages on the first day and 100 pages on the second day, then on the first day, he did read 20 pages more so 20/100= 20% more on the first day than on the second day. But 20% of 120 is 24 pages so "20% less than on the first day would be 120- 24= 96, not 100.