Background Info: A radioactive isotope is an element that has a nucleus that is so unstable, that
it causes the substance to decay into a different
element. The half-life of a radioactive isotope is how
long it will take for that substance's mass to
decrease by half. Explained differently, after 1 half-

life period of time, 50% of the substance is gone.
The exponential decay equation used to determine
how much of a substance that is left, y, is:
a
a is the starting mass
x is the number of half-lives that have passed.
(EX. If the half life is 3 days, then 6 days would be 2
half lives and 9 days would be 3 half-lives)
y = a(0.5)*
=
Question 9 (1 point)
Arsenic-74 is used to locate brain tumors. It has a
half-life of 17.5 days. 90 mg were used in a
procedure. Write an equation that can be used to
determine how much of the isotope is left after x
number of half-lives.

Please help me it’s so confusing!!