Chapter 4, Exponential and Logarithmic Functions - Section 4.5 - Exponential and Logarithmic Functions - 4.5 Exercises - Page 405: 98

a) $7.87$ lumens b) $86.6$ ft

a) Plugging in $x=30$, we get: $I=10e^{-0.008(30)}\approx7.87$ lumens b) Plugging in $I=5$ into the equation, we obtain: $5=10e^{-0.008x}\\e^{-0.008x}=0.5\\\ln(0.5)=-0.008x\\x=\frac{\ln(0.5)}{-0.008}\approx86.6~ft$