Simple division provides us with quotient like 15/6 will give 2 in output.
MOD formula gives the remainder of divison. For example 15/6 will give 3 as output.

MOD is represented by % symbol in programming languages.

MOD formula is helpful in many situation like

1. when we are looking to find a series of numbers which are divisible by certain number. For example we have a list of roll numbers of students and needed to divide students in 4 groups. So we can divide each roll number with 4 and remainder of divison will decide the group of student. 13 MOD 4=1 is assigned group 1, similary 15 MOD 4=3 will be assigned group 3.

2. whenever we have to take decision about number on the basis of their last digit
In this scenario we can use Number MOD 10=last digit. Accordingly we can take the decision.

Lets us illustrate this using MS Excel.
Fig 1 show the Roll Number and corresponding groups using the MOD formula.

Fig 1

But there is one exception in MOD formula. It fails when number is greater than 10 digits long.

But there is simple mathematical solution to this

=RollNumber-(INT(RollNumber/divisor)*divisor)

This formula can replace the MOD formula and work in every case.
Lets apply