The ratio of speed of boat in still water to speed of stream is 8 : 1. It takes 4 hours by boat to cover 54 km in downstream & 42 km in upstream. Find the downstream speed of boat.

let speed of boat in still water & stream be 8x kmph & x kmph respectively

ATQ, $frac{{54}}{{8x + x}} + frac{{42}}{{8x - x}} = 4$

$frac{6}{x} + frac{6}{x} = 4 Rightarrow x = 3$

Downstream speed = 8x + x = 27 kmph