A 15.0- resistor and a coil are connected in series with a 6.30-V battery with negligible internal

Question:

A 15.0-Ω resistor and a coil are connected in series with a 6.30-V battery with negligible internal resistance and a closed switch.
(a) At 200 ms after the switch is opened the current has decayed to 0.210 A. Calculate the inductance of the coil.
(b) Calculate the time constant of the circuit.
(c) How long after the switch is closed will the current reach 1.00% of its original value?