$t_2 = t - t_1 = 1 - \dfrac{1}{4}t = \dfrac{3}{4}t( h ) $
$v_{tb} = \dfrac{S_1 + S_2 }{t_1 + t_2 }$
$⇔ 48 = \dfrac{ v_1 . t_1 + v_2 . t_2 }{ t }$
$⇔ 48 = \dfrac{ v_1 . \dfrac{ 1}{4}t + v_2 . \dfrac{3}{4}t }{ t }$
$⇔ 48 = \dfrac{t( v_1 . \dfrac{1}{4} + \dfrac{3v_2}{4 })}{t}$
$⇔ 48 = \dfrac{ v_1 + 3v_2 }{4 }$
$⇒ v_2 = 54 km/h $