After how many half lives a radioactive substance will decay to 6.25% of its original value:

Four half lives

The correct answer is Option (4) → Four half lives

Remaining amount of a substance after n half-lives -

$\left(\frac{1}{2}\right)^n×100\%=6.25\%$

$\left(\frac{1}{2}\right)^n=0.0625$

$⇒\left(\frac{1}{2}\right)^n=\left(\frac{1}{2}\right)^4$

n = 4 half life