Question 1060126
Same problem, different numbers.
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5^(-1x+1) = 15
 (1-x)*log(5) = log(15)
 log(5) - x*log(5) = log(15)
 x*log(5) = log(5) - log(15) = log(1/3)
 x = log(1/3)/log(5)
 x =~ -0.6826