Here is the solution
k = 2.303 /t * log [a] / [a-x]
In the given case a=100 and X=90
Therefore
k = 2.303 /30 * log 100/ 10 .......a
Let the be the required time then
k = 2.303 /t * log 100 / 0.1 .......b
Equating a and b we get
(log 100/ 10) /30=(log 100 / 0.1)/t
Therefore t= [(log 100 / 0.1)/(log 100/ 10)]30
t=90 min
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