Solution
For this case we know the initial value 80 and the half life if 5 seconds
Then we can create the following equation:
A = 80 (1/2)^(t/5)
Then we can replace for each case like this:
A
[tex]A(5)=80\cdot(\frac{1}{2})^1=40[/tex]B
[tex]A(10)=80\cdot(\frac{1}{2})^2=20[/tex]C
[tex]A(15)=80\cdot(\frac{1}{2})^3=10[/tex]D
[tex]A(20)=80\cdot(\frac{1}{2})^4=5[/tex]E
[tex]A(25)=80\cdot(\frac{1}{2})^5=\frac{5}{2}=2.5[/tex]