first convert 3 and 1/2 to imporoper
3 and 1/2=6/2+1/2=7/2 feet
convert to inches
7/2 times 12=84/2=42 inches
k so
total space used=number of books times space each book uses
total space possible=42
space each book uses=5/8
so
42=numberofbooks times 5/8
multiply both sides by 8/5 to clera fraction (5/8 times 8/5=40/40=1)
336/5=number of books
67.2=number of books
since you cannot fit 0.2 book (besides being useless or very small), we round down
67 books