An inductor of self-inductance L=20 H is connected in series with a resistor of resistance `R = 4 Omega`, key and battery of emf E. Key is closed at time t = 0. How long will it take for magnetic energy to reach one-fourth of its maximum value?

Explanation

To solve the problem, we need to determine how long it takes for the magnetic energy stored in the inductor to reach one-fourth of its maximum value after the key is closed.

### Step-by-Step Solution:

1. **Identify Given Values:**
- Self-inductance, \( L = 20 \, \text{H} \)
- Resistance, \( R = 4 \, \Omega \)
- Maximum current, \( I_0 = \frac{E}{R} \) (where \( E \) is the emf of the battery)
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