what is a function? if the input is 72 what is the output

a function is basically a rule that relates an input value to an output value. There can be only one output value for any input value.

So, the table
1 4
3 9
0 2
is a function, but
1 4
4 8
1 5
is not, since there's no way to determine which value corresponds to 1.

You have given us an input value, but there's no way to tell the output unless you provide a table as above, or a rule connecting input values to output values.

For example, if the rule is

f(x) = x-20
then
f(72) = 72-20 = 52

if the input is 24 what is the output?

the table is: the input table:
x,4,8,12,16
the output table is:
y,1,2,3,4

9

A function is a mathematical term that describes a relationship between a set of inputs and a set of outputs, where each input corresponds to exactly one output. In simpler terms, it is a rule or a set of instructions that takes a value as input and produces a corresponding value as output.

To determine the output of a function when the input is given, we need to know the specific function or the rule it follows. Without further information, it is not possible to determine the output of the function when the input is 72.