Chapter 7 Exponential and Logarithmic Functions - 7.5 Apply Properties of Logarithms - 7.5 Exercises - Problem Solving - Page 512: 69

$\approx76.2$

$L(I)=10\log\frac{I}{I_0}$ where $I_0=10^{−12}$. The three conversations have a combined intensity of $I=3\cdot1.4\cdot10^{-5}=4.2\cdot10^{-5}$ Hence here $L(I)=10\log\frac{4.2\cdot10^{-5}}{10^{−12}}=10\log{4.2\cdot10^7}\approx76.2$