(b) 4 km/hr
Let the speed downstream be x km/hr and speed upstream be y km/hr. Then,
\(\frac{28}{x}+\frac{24}{y}=6\) .....(i)
and \(\frac{21}{x}+\frac{30}{y}=\frac{13}{2}\) ....(ii)
Multiplying (i) by 3 and (ii) by 4 and then subtracting eqn (i) from eqn (ii), we get
Putting the value of y in (i), we have
\(\frac{28}{x}+4=6\) ⇒ \(\frac{28}{x}=2\) ⇒ x = 14 km/hr
∴ Speed of the stream = \(\frac12(14-6) \) km/hr = 4 km/hr.
