Let
x-------> the total number of empty spaces in the bike racks
y------> the total number of spaces in the bike racks
z-------> the total number of bike in the bike racks
we know that
\(x=y-z\\ y=15*4=60\\ z=16\)
\(x=y-z\\ x=60-16\\ x=44\)
therefore
the answer is
the total number of empty spaces in the bike racks is \(44\)
the expression is equal to
\(x=4*15-16\)