## Explanation

Data validation rules are triggered when a user adds or changes a cell value. When a custom formula returns TRUE, validation passes and the input is accepted. When the formula returns FALSE, validation fails and the input is rejected.

In this case, the MOD function is used to perform a *modulo operation*, which returns the remainder after division. The formula used to validate input is:

```
=MOD(C5,100)=0
```

The value in C5 is 500. The MOD function divides 500 by 100 and gets 5, with a remainder of zero. Since 0 = 0, The rule returns TRUE and the data validation passes:

```
=MOD(500,100)=0
=0=0
=TRUE
```

If a user enters, say, 550, the remainder is 50, and validation fails:

```
=MOD(C5,100)=0
=MOD(550,100)=0
=50=0
=FALSE
```

*Note: Cell references in data validation formulas are relative to the upper left cell in the range selected when the validation rule is defined, in this case C5.*