e = 4 x 15 - 16
Let's create an expression.
We have 4 bike racks
e = 4
Each bike rack can hold 15 bikes
e = 4 x 15
There are 16 bikes in the bike racks
e = 4 x 15 - 16
So the value e will be the total number of empty spaces in the bike racks. The overall expression is
e = 4 x 15 - 16