Answer
rowing speed: 5.25 mph, current speed: 2.75 mph
Work Step by Step
10 miles = (x+y)*1.25 (downriver)
10 miles = (x-y)*4 (upriver)
$10=(x+y)*1.25$
$10=(x-y)*4$
$10=(x+y)*1.25$
$1.25x+1.25y=10$
$10=(x-y)*4$
$4x-4y=10$
$1.25x+1.25y=10$
$4x-4y=10$
$1.25x+1.25y=10$
$4*(1.25x+1.25y=10)$
$5x+5y=40$
$4x-4y=10$
$5/4*(4x-4y=10)$
$5x-5y=12.5$
$5x+5y=40$
$5x-5y=12.5$
$5x-5y+5x+5y=12.5+40$
$10x=52.5$
$10x/10=52.5/10$
$x=5.25$
$10=(x-y)*4$
$10=(5.25-y)*4$
$10/4=(5.25-y)*4/4$
$2.5=5.25-y$
$y+2.5=5.25$
$y=2.75$