Answer
$ \approx 6\times10^5 books $.
Work Step by Step
Looking at a shelved textbook, assume it is 0.05m wide and 0.25m deep.
With books on 2 sides, the shelf is about 0.50m deep. If the aisle is 1.5 m wide, then a fraction of (0.50m)/(1.5m+0.50m), or a quarter of the floor space, is taken by shelving units.
Find the number of books on a single shelf level in the whole library.
$(0.25)(3500 m^2)(\frac{1 book}{(0.05m)(0.25m)})=7.0\times10^4 books$
There are 8 rows of shelves, top to bottom (this seems excessive, but it is the information given). The total number of books stored is 8 times what we just calculated. Rounding off to one significant figure, the library contains $(8)(7.0\times10^4 books) \approx 6\times10^5 books $.
