To row a certain distance upstream and downstreamtakes $$t_1$$ hours and $$t_2$$ hours respectively and speed of stream is 'y' km/h. Find speed
in still water.

A

$$ \left(\frac {t_1  ^2 - t_2  ^2} { 2y}\right) km/h $$

B

$$ \left[\left(\frac {t_1   - t_2 } { t_1  + t_2}\right)  y \right ]km/h $$

C

$$ \left[ \frac{(t_1   + t_2 ) 2y}{ ( t_1  - t_2)} \right ]km/h $$

D

$$ \left[ \frac{(t_1   + t_2 )}{ ( t_1  - t_2) }y \right ]km/h $$