Based on table N of the reference tables what is the number of hours required for 42k (potassium -42) to undergo three half-life periods

The number of hours required : 37.2 hours

Given

⁴²K (potassium -42)

Required

The number of hours

Solution

The atomic nucleus can experience decay into 2 particles or more due to the instability of its atomic nucleus.  

Usually, radioactive elements have an unstable atomic nucleus.  

Based on Table N(attached), the half-life for ⁴²K is 12.4 hours, which means half of a sample of ⁴²K will decay in 12.4 hours

For three half-life periods :

the formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula

(that's what i know about formula mass)