A boat covers 48 km upstream and 72 km downstream in 12 hours, while it covers 72 km upstream and 48 km downstream in 13 hours The speed of stream is :

Explanation:-

Let downstream speed be D and Upstream speed be U. Then
(⁴⁸/_D + ⁷²/_U) = 12 -------- (1)
and
(⁴⁸/_D + ⁷²/_U) = 13 ------------ (2) On solving the eqn (1) and (2), we get
D = 12 kmph and
U = 8 kmph
Thus, speed of the current = ^(D-U)/₂ = ⁴/₂ = 2 kmph.