a boat can travel 140 miles downstream in the same time that it takes to travel 92 miles upstream. The speed of the current is 6mph. What is the speed of the boat in still water?

Question

dilla · Answer

@triciaal

triciaal · Answer

@mathmate please help  I'm about to leave

mathmate · Answer

@triciaal  Sorry, I wasn't online.
Let the boat speed in stillwater be S, and speed of current be C.
Then 
S+C=downstream speed
S-C=upstream speed
Use Distance = Speed * Time
=> time = Distance / Speed

We are given time to travel 140 miles downstream is the same as time to travel 90 miles upstream, which makes
140/(S+C)=90/(S-C)
Since we know the current speed C=6,
above simplifies to
140/(S+6)=90/(S-6)
Cross multiply:
140(S-6)=90(S+6)
solve for S.