A person can row a distance of 3\(\frac{3}{4}\) km upstream in 1\(\frac{1}{2}\) hours and a distance of 13 km downstream in 2 hours. How much time ( in hours) will the person take to row a distance of 90 km in still water? 

20

(U.S) Upstream speed = \(\frac{3\frac{3}{4}}{1\frac{1}{2}}\)

                                 = \(\frac{\frac{15}{4}}{\frac{3}{2}}\) = \(\frac{5}{2}\) km/hr

(D.S) Downstream speed = \(\frac{13}{2}\) km/hr

Speed of boat (in still water) = \(\frac{D.S + U.S}{2}\)

                                          = \(\frac{\frac{13}{2} + \frac{5}{2}}{2}\) = 4.5 km/hr

Time taken to cover a distance of 90 km = \(\frac{90}{4.5}\) = 20 hours