The time constant of and R- C cirucit during charging is them time in which the charge on the condenser plates, as compared to maximum charge `(q_0)` becomes `(q//q_0)xx `100,` which is equal to …….. .

Explanation

To solve the problem, we need to determine the time constant of an R-C circuit during charging, specifically the time at which the charge on the capacitor plates becomes a certain percentage of the maximum charge \( Q_0 \).

### Step-by-Step Solution:

1. **Understand the Charging Equation**:
The charge \( Q \) on a capacitor during charging in an R-C circuit is given by the equation:
\[
Q = Q_0 (1 - e^{-t/\tau})
...