Choose number 2, it is divisible by 2,

but it is not divisible by 4, because

but

.
It will be 1/4.
That is your answer.
Hope this helps
I'm pretty sure you are asking for the value of b. Let's actually write it and then solve it:

Solving for b yields:

So,
b = 19.59. If you want the answer in simplified radical form, we can also do that:

So,
b = 8√6 = 19.59