9720, 3240, 1080, 360, 180, 40
Find the wrong number in the given series.

To find the wrong number in the given series, let us analyze the mathematical pattern between consecutive terms.

Given series: 9720, 3240, 1080, 360, 180, 40

Let us check the division/multiplication rule moving from left to right:

$9720÷3=3240$

$3240÷3=1080$

$1080÷3=360$

Following this logic of dividing each term by 3 to get the next term:

$360÷3=120$

If the fifth term were 120 instead of 180, then checking the final term:

$120÷3=40$

This matches the final term (40) in the given sequence. Therefore, 180 does not fit the pattern and is the wrong number in the series. It should be replaced by 120.

Thus, the correct option is 180.