Answer: t = 0.492τ
Explanation: In a circuit where there is a resistor and a capacitor, the equation for a charging capacitor is given by:

where:
is the equilibrium charge
q is the charge at time t
RC is time constant also called τ (tau)
For this problem, the circuit is charged to 39%, which means: q =


0.39 = 1 -

= 0.61
= ln(0.61)
-t = ln(0.61)τ
t = 0.492τ
For the condition to be met it is needed 0.492 time constants must elapse.