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## 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