Upstream and downstream boat speed

Let upstream speed be $u$ and downstream speed be $v$. $\frac{24}{u} + \frac{72}{v} = 8$ $\frac{48}{u} + \frac{108}{v} = 14$

Multiplying the first equation by 2: $\frac{48}{u} + \frac{144}{v} = 16$

Subtracting the second equation: $\frac{36}{v} = 2 \implies v = 18\text{ km/h.}$ Substitute $v = 18$ into first equation: $\frac{24}{u} + 4 = 8 \implies u = 6\text{ km/h.}$

Speed of boat in still water = $\frac{18 + 6}{2} = 12$ km/h. Speed of stream = $\frac{18 - 6}{2} = 6$ km/h.