N = 75,000*(1/2)^(h/4) . . . . N = number alive; h = hours
3000 = 75000*(1/2)^(h/4) . . . . . substitute given numbers
1/25 = (1/2)^(h/4) . . . . . . . . . . . . . .divide by 75000
log(1/25) = (h/4)*log(1/2) . . . . . . . take the log, then divide by the coefficient of x
h = 4*log(1/25)/log(1/2) ≈ 18.6 . . . . hours
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Of course log(1/25) = -log(25) = -2log(5), so the expression for h can be written as h = 8log(5)/log(2)
