If there are 20 units of hormone in the bloodstream and it's half life is 2 hours how much will be left after 4 hours

Half-life is a time that needed by a substance to decay half of its mass. The hormone initial number is 20 unit and its half-life is 2 hour. Then, the number of hormone after 4 hours would be:

final number= initial number / 2^(duration/half-life
final number= 20 units / 2^(4 hour/2 hour)
final number= 20 units / 2^2
final number= 20 units / 4= 5 units