Hi guys, I am doing A-levels and I came across two question I need help with.

We are told the exponential equation isP= ab^t

P=Quantity

a= initial value

b=constant indicating how fast the quantity is growing

t=time

I know all but one value for the equation P= ab^-t, which is a. How Do I know the initial value of this substance?Question 1)A radioactive substance decays at rate of 12% per hour

a)Find after how many hours half of the radioactive material will be left.

b) How many hours did it have twice the current amount of radioactive material?

I have no idea how to do this one.Question 2)

A dangerous radioactive substance has a half life of 90 years. It will be deemed safe when its activity is down to 0.005 of it's initial value. How long before it's deemed safe?

Thanks in advance.