Solve the Problem $\mathrm { v } = 156 \cos \left( 120 \pi \mathrm { t } - \frac { \pi } { 3 } \right)$

Multiple Choice

Solve the problem.
-The output voltage for an a.c. generator is approximated by $\mathrm { v } = 156 \cos \left( 120 \pi \mathrm { t } - \frac { \pi } { 3 } \right)$ . Find the smallest positive value of $t$ for which the output is 68 volts. (Use $3.1416$ as an approximation for $\pi$ .)

A) $t = 0.0124$
B) $t = 0.0114$
C) $\mathrm { t } = 0.0047$
D) $t = 0.0057$