Answer
$$t=\frac{1}{5}\ln{\frac{100}{7}}$$
Work Step by Step
$7e^{5t}=100$
Dividing both sides by 7:
$e^{5t}=\frac{100}{7}$
Taking the natural log of both sides:
$5t=\ln{\frac{100}{7}}$
Solving for $t$:
$t=\frac{1}{5}\ln{\frac{100}{7}}$