How Long Until Superman Regains His Powers?

[Yohan Lee]

Homework Statement

superman has been disabled by a nearby amount of kryptonite, which decays exponentially. If Superman cannot regain his power until 90% og the kryptonite disintegrates, then how long will it be before he regain his powers?
Use r=-0.138629. Round to the nearest day.
a. 4days c 11days
b 6days d 17days e 21days

Homework Equations

Q(t) = Qe^(rt)

The Attempt at a Solution

i tried 0.9Q=Qe^(-0.138629t)
0.9=e^(-0.138629t)
ln0.9=lne^(-0.138629t)
ln0.9=-0.138629t
ln0.9/-0.138629 =t
t=0.760018
and i was not able to find the answer

 

[Bystander]

If 90% has decayed, how much is left?

 

[Yohan Lee]

Thank you i got 0.1Q=Qe^(-0.138629t)
t= 16.6