Suppose there are 100,000 atoms of a radioactive substance that has a ½ life of 10 minutes. How many atoms will remain after 40 minutes: a. 2,500
b. 10,000
c. 12,500
d. 50,000

Show calculation

c. 12,500

Explanation:

Original number of atoms = 100,000 atoms

Half- life  = 10min

Unknown:

The number of atoms that will remain after 10min  = ?

Solution:

The half - life is the time taken for half of a radioactive substance to decay by half.

    Time taken        Number of atom   half life

           10min             100000                   _

           20min             50000                    1

           30min             25000                    2

           40min              12500                     3

this would be about 43%