Jane arranges m chairs in each of n rows to seat x people.
the formula to find the number of chairs in each row is m =

if 60 people attend the meeting and each row has 4 chairs, there are
rows.

The formula to find the number of chairs in each row is m = x ÷ n,
where
m - the number of chairs,
n - the number of rows,
x - the number of people.

If 60 people attend the meeting and each row has 4 chairs, there are 15 rows.
This is assumed using the above formula:
m = x ÷ n

If:
x = 60
m = 4

Then:
4 = 60 ÷ n
⇒ n = 60 ÷ 4
⇒ n = 15

To evaluate , you'd start by rewriting this as and your first step of evaluation is to evaluate .

When you evaluate , you have the following:

    

and this is undefined.  Since you cannot perform the first step of the evaluation, you cannot evaluate .