The calculation of the time constant $\tau$ for an RL circuit requires the values of Inductance ($L$) and the equivalent Resistance ($R_{eq}$) of the circuit ($au = L/R_{eq}$). The question refers to a figure which is not provided in the source material. While the concept that the quantity $L/R$ has the dimensions of time is covered in the NCERT text , the specific numerical problem cannot be solved without the circuit diagram.