The​ half-life of a certain substance is years. How long will it take for a sample of this substance to decay to ​% of its original​ amoun?

Answer 1

One day.  Decay happens daily; so in1 day the substance will be at 99.999--% considering you did not specify an amount. 

Answer 2

Why do middle-school science questions keep posting in the finance section?

Anyway, a half-life is how long it takes for half of a substance to decay. If the half-life is 7 years, then after 7 years there is 1/2 the original amount, then after another 7 years (14 from start) there is 1/4 of the original amount, and so on.

Final amount = (Original amount) x (1/2)^(number of half-lives)

This equation works with non-integer values. If the half-life is 7 years, then after 4 years there is (1/2)^(4/7) of the original amount.

And of course you can rearrange the equation to solve for a different variable.

Original amount = (Final amount) x 2^(number of half-lives)

number of half-lives = log_2 ((Original amount)/(Final amount))

Answer 3

unrelated to personal finance,CHEATER