The current in a bayou is 4 miles per hour. A boat travels 48 miles upstream in the same amount of time it takes to travel 72 miles down stream

if speed of boat in still water is x then atts time is same in both cases 

ie. 48/x-4= 72/ x+4

solving this: 48x+ 4.48=72x-4.72

so 24x= 120.4

so x= 5.4=20 ans