Chapter 7: Transcendental Functions - Section 7.4 - Exponential Change and Separable Differential Equations - Exercises 7.4 - Page 401: 28

$\approx 92.1 s$

As we are given that when the voltage is 10% of its original value this means that we have $V =0.1 V_0$ Thus, $V(t)=V_0e^{-t/40}$ $(0.1) V_0=V_0e^{-t/40}$ Also, $t=-40 [\ln (0.1)] \implies t \approx 92.1 s$ Thus, $t \approx 92.1 s$