Explanation:

Let’s break this series down:

- Here’s the pattern:

18, 3, 36, 6, 54, 9, ?

- Look at the jumps.

18 to 3 ? divide by 6

3 to 36 ? multiply by 12

36 to 6 ? divide by 6

6 to 54 ? multiply by 9

54 to 9 ? divide by 6

- So, it seems:

Alternating between “divide by 6” and “multiply by a growing number” (12, 9... see a pattern dropping by 3 each time?).

Now, keep this going:

After 9, next should be multiply by 6 (the next in the “multiply by” series: 12, 9, 6...)

So, 9 × 6 = 54

But wait! That goes back to an earlier number (there’s a loop?).

Still, checking the options:

- 1. 89 – not fitting the pattern.

- 2. 90 – doesn’t fit the multiplication pattern.

- 3. 12 – not matching the multiply by trend.

- 4. 72 – what if the multiplication pattern isn’t dropping by 3, but always ×6? Let’s review:

- 18 ÷ 6 = 3

- 3 × 12 = 36

- 36 ÷ 6 = 6

- 6 × 9 = 54

- 54 ÷ 6 = 9

- 9 × 8 = 72 (maybe the multiply pattern is 12, 9, 8 ... decreasing by 1 less each time?)

So 72 fits if you pick up that multiplying number as 12, 9, 8 (each time dropping by 1 less, from 3 to 1).

- Option 4: 72