a radioactive substance decays 10% in 5 days. The amount remains after 20 days?

Answer 1

You need to determine k, the rate constant and I would do it this way.
ln(No/N) = kt
No = 10
N = 90
k = solve for this
t = 5 days.

Then knowing k, redo
ln(No/N) = kt
No = 100
N = Solve for this.
k from above
t = 20 days

Answer 2

How