What is the half life of a substance if it decays by 75% in two weeks?
since it decays by 75% in two weeks, it has reduced to 1/4.
So, it reduces to 1/2 in 1 week.
half-life is one week.
Or, setting up aquation where t = no. of weeks,
A = Ao (1/4)^(t/2)
when is it at 1/2 Ao?
1/2 = (1/4)^(t/2)
square both sides:
1/4 = (1/4)^t
t = 1