https://www.quora.com/What-is-the-next-number-in-the-series-81-54-36-24

We get next term in sequence by $\left(\dfrac{X}{3}\right)\times 2$ $\dfrac{81}{3} =27\times 2 = 54$

$\dfrac{54}{3} =18\times 2 =36$

$\dfrac{36}{3} =12\times 2 = 24$

$\dfrac{24}{3} = 8\times 2 = 16$

Answer)16

@Rupendra Choudhary,

Yes I approached it in the same way.

ALTERNATE LOGIC (Look at the number of 2's) $ 81 = 3 \times 3 \times 3 \times 3 $

(Look at the number of 2's)

$ 54 = 3 \times 3 \times 3 \times$$ 2 $

$ 36 = 3 \times 3 \times$$ 2 \times 2 $

$24= 3 \times$$ 2 \times 2 \times 2 $

$16=$ $2 \times 2 \times 2 \times 2 $