The speed of the stream: v s = 1 mph v - David`s speed in still water ( v - 1 ) t 1 = 3 ( v + 1 ) t 2 = 3 t 1 + t 2 = 4 --------------------------- t 2 = 4 - t 1 3 = ( v - 1 ) * t 1 3 = ( v + 1 ) * ( 4 - t 1 ) ----------------------------- 3 = v t 1 - t 1 + 3 = 4 v - v t 1 + 4 - t 1 ------------------------------ 6 = 4 v + 4 - 2 t 1 2 = 4 v - 2 t 1 2 t 1 = 4 v - 2 / : 2 ( divide both sides of equation by 2) t 1 = 2 v - 1 t 2 = 5 - 2 v ------------------ 3 = ( v - 1 ) * ( 2 v - 1 ) 3 = 2 v² - 3 v + 1 2 v² - 3 v - 2 = 0 v 1/2 = 3+/- √(9 + 16 ) / 4 = (3 + 5)/4 = 8/4 = 2 ( other solution is negative ) Answer: David`s speed in still water is 2 miles per hour.
