The halif life of a given radioactive nuclide in 138.6 days. How much will a sample of tis radioactive nuclide gets reduced to only 12.5% of its initial value?

@ankuraggarwal: Correct!

It will take 3 half lives because it will go from

100% to 50% in one half life.

50% to 25% in second half life.

25% to 12.5% in third half life.

You can see that the quantity is getting halved each time.