For a geometric series Sn = a1 (1-rn) / (1-r)
The common ratio is 1.5 (1.5 more pages each day) a1 = 5
80 = 5 (1- 1.5n) / (1-1.5)
-40 = 5 (1-1.5n )
-8 = (1-1.5n)
-9 = -1.5n
9 = 1.5n Take log of both sides
log9/log1.5 = n = 5.41 days ~~~ 6 days to read the 80 page book.