The key is to remember the bottom-changing formula and the inference of the bottom-changing formula.

If you can't remember, here's the solution:

Order: log9 (16) = X.

Then: the x power of 9 = 16.

The second power of 3 = the fourth power of 2.

Then both sides take the logarithm with base 3 at the same time to get:

2x=4log3(2)

x=2log3(2)

The same method can also be used to get: log8(8 1)=4/3log2(3) (but this time I take the logarithm with base 2 on both sides, and of course I can also take the logarithm with base 3).

So: log9 (16) * log8 (81) = 2log3 (2) × 4/3× log2 (3) = 8/3.