The idea with nuclear half-life calculations is that the initial mass of the isotope will be halved with every passing of the isotope's half-life.
If you initial amount of fluorine-21 is
#A_0/2 ->#after one half-life;
What will happen after a second half-life passes?
Well, the amount will be halved again, so you know that you will be left with
#(A_0/2)/2 = A_0/2 * 1/2 = A_0/2^2 ->#after two half-lives
What about after another half-time passes?
#A_0/4 * 1/2 = A_0/2^3 ->#after three half-lives
The pattern is very clear at this point - to get the amount of a radioactive isotope that remains after
#A_n = A_0/2^color(blue)(n)#
In your case, the half-life of fluorine-21 isotope is 5 seconds. How many half-lives must pass for one minute to pass?
#1color(red)(cancel(color(black)("min"))) * (60color(red)(cancel(color(black)("s"))))/(1color(red)(cancel(color(black)("min")))) * "1 half-life"/(5color(red)(cancel(color(black)("s")))) = "12 half-lives"#
This means that after one minute passes you will be left with
#A_"1 minute" = A_0/2^12 = A_0 * 1/4096#