There are 40 students in a mathematics class. If 5 students were absent on Monday, what percentage of the class was absent on that day?

The number of students absent on Monday is 5.

So, the total number of students present on Monday is 40 - 5 = <<40-5=35>>35.
Therefore, the percentage of the class that was absent on Monday is (5/40) x 100% = <<5/40*100=12.5>>12.5%. Answer: \boxed{12.5}.