In a 100g sample of P³² there are 100/32 × N no of atoms . ( N is the avogadro no.)
In a 25g  sample of N¹⁶ there are  25/16 ×N   no of atoms .
so from observation 25/16 is half of 100/32 
so the no of atoms in the sample reduced to half of the initial value in 28.6 days
so half life of the sample is 28.6 DAYS