Question: this is a question from introduction to specialist maths any...

This is a question from Introduction to Specialist Maths, any method that is easy to understand would be best.

Iodine-131 has a half life of 8 days. It is used as a radioisotope to track some types of cancer.
Calculate the amount of Iodine-131 left in the body after 47 days if the initial injection had 100mg of
Iodine-131.