A radio active substance decays at a particular rate such that 0.020 g is left after 5 days and 0.015 g is left after 10 days. What is the half-life of this substance

12 days = Half-life

Explanation:

The radioactive decay for an isotope A follows the equation:

Ln[A] = -kt + Ln[A]₀

Where [A] is amount of A after time t, k is decay constant and [A]₀ is initial amount of the isotope.

Replacing:

Ln[A] = -kt + Ln[A]₀

Ln[0.015g] = -k*5days + Ln[0.020]

-0.2877 = -k*5days

0.0575days⁻¹ = k

And we can obtain Half-Life from decay constant as follows:

Ln 2 / k = Half-life

Ln 2 / 0.0575days⁻¹ = Half-Life

i think its 100.

hope this !