<PRE><B>Question 307831</B>
a motor boat has a four-hour supply of gasoline. How far fom the marina can it travel if the rate going out against the current is 20mi/h and the rate coming back with the current is 30mi/h

let it go x miles and return x miles 
while going the time taken = x/ 20 hours

.
while returning it takes x/30 hours

x/20+x/30 = 4 hours 

LCM = 60

3x+2x / 60 = 4
5x= 60*4
5x= 240
x= 48 miles

It can 48 miles from the marina</PRE>