**We know the Current formula in L/R circut**

I(t) = I_{o} [1 - e^{-t/τ} ]

, where I_{o} = ε/R is the maximum current

and the time constant τ = L/R

if current reaches 99 % of steady state, then

[1 - e^{-t/τ} ]

= 0.99 or e^{-t/τ}

= 0.01 = 10^{-2} .

**Hence t/τ = 2 × ln10 = 4.6**

if current reaches 99.9 % of steady state, then

[1 - e^{-t/τ} ]

= 0.999 or e^{-t/τ}

= 0.001 = 10^{-3} .

**Hence t/τ = 3 × ln10 = 6.9**