Two substances \( \mathrm{A}\left(\mathrm{t}_{1 / 2}=5\right. \) mins \( ) \) and \( \mathrm{B}\...

Two substances \( \mathrm{A}\left(\mathrm{t}_{1 / 2}=5\right. \) mins \( ) \) and \( \mathrm{B}\left(\mathrm{t}_{1 / 2}=15\right. \) mins \( ) \) are taken is such a way that initially \( [\mathrm{A}]=4[\mathrm{~B}] \). The time after which the conentration of both will be equal is
(a) \( 5 \mathrm{~min} \)
(b) \( 15 \mathrm{~min} \)
(c) \( 20 \mathrm{~min} \)
(d) Concentration can never be equal