SOLUTION: I am not sure how to set this problem up. Can you help? You travel 5 miles in a boat. because of a 4 mph current it took you 20 minutes longer to go then return. How fast will yo

Algebra ->  Quadratic Equations and Parabolas -> SOLUTION: I am not sure how to set this problem up. Can you help? You travel 5 miles in a boat. because of a 4 mph current it took you 20 minutes longer to go then return. How fast will yo      Log On


   



Question 193861: I am not sure how to set this problem up. Can you help?
You travel 5 miles in a boat. because of a 4 mph current it took you 20 minutes longer to go then return. How fast will you go without current?
Is the problems set up like-
10x+100=x^2+20x

Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
The equation is
distance = rate x time but with a modification:
Let r%5Bs%5D= rate of the boat in still water in mi/hr
Let r%5Bc%5D= rate of the current in mi/hr
Let t%5Bu%5D= time going upstream in hrs
Let t%5Bd%5D= time going downstream in hrs
r%5Bs%5D+-+r%5Bc%5D= rate of boat against the current in mi/hr
r%5Bs%5D+%2B+r%5Bc%5D= rate of the boat going with the current in mi/hr
-------------------
So, going upstream, I can write:
(1) d+=+%28r%5Bs%5D+-+r%5Bc%5D%29%2At%5Bu%5D
And going downstream, I can write:
(2) d+=+%28r%5Bs%5D+%2B+r%5Bc%5D%29%2At%5Bd%5D
-------------------
Given:
d+=+5 mi
t%5Bu%5D+=+t%5Bd%5D+%2B+20 hrs
r%5Bc%5D+=+4 mi/hr
-------------------
Now I can rewrite (1) and (2)
(1) 5+=+%28r%5Bs%5D+-+4%29%2A%28t%5Bd%5D+%2B+20%29
(2) 5+=+%28r%5Bs%5D+%2B+4%29%2At%5Bd%5D
This is 2 equation with 2 unknowns, so it's solvable
(1) 5+=+%28r%5Bs%5D+-+4%29%2A%28t%5Bd%5D+%2B+20%29
(1) 5+=+r%5Bs%5D%2At%5Bd%5D+-+4t%5Bd%5D+%2B+20r%5Bs%5D+-+80
(1) r%5Bs%5D%2At%5Bd%5D+%2B+20r%5Bs%5D+=+4t%5Bd%5D+%2B+85
(1) r%5Bs%5D%2A%28t%5Bd%5D+%2B+20%29+=+4t%5Bd%5D+%2B+85
And, from (2)
(2) 5+=+%28r%5Bs%5D+%2B+4%29%2At%5Bd%5D
(2) 5+=+r%5Bs%5D%2At%5Bd%5D+%2B+4t%5Bd%5D
(2) r%5Bs%5D%2At%5Bd%5D+=+-+4t%5Bd%5D+%2B+5
(2) r%5Bs%5D+=+-4+%2B+5%2Ft%5Bd%5D
substitute this in (1)
(1) %28-4+%2B+5%2Ft%5Bd%5D%29%2A%28t%5Bd%5D+%2B+20%29+=+4t%5Bd%5D+%2B+85
(1) -4t%5Bd%5D+%2B+5+-+80+%2B+100%2Ft%5Bd%5D+=+4t%5Bd%5D+%2B+85
(1) 8%2A%28t%5Bd%5D%29%5E2+%2B+160t%5Bd%5D+-+100+=+0
(1) 2%2A%28t%5Bd%5D%29%5E2+%2B+40t%5Bd%5D+-+25+=+0
Using quadratic equation:
t%5Bd%5D+=+%28-b+%2B-+sqrt%28+b%5E2-4%2Aa%2Ac+%29%29%2F%282%2Aa%29+
a+=+2
b+=+40
c+=+-25
t%5Bd%5D+=+%28-40+%2B-+sqrt%28+40%5E2-4%2A2%2A%28-25%29+%29%29%2F%282%2A2%29+
t%5Bd%5D+=+%28-40+%2B-+sqrt%28+1600+%2B+200+%29%29%2F4+
t%5Bd%5D+=+%28-40+%2B-+42.426%29%2F4+
t%5Bd%5D+=+2.426%2F4
t%5Bd%5D+=+.6065 hrs
I want to find r%5Bs%5D
(2) r%5Bs%5D+=+-4+%2B+5%2Ft%5Bd%5D
(2) r%5Bs%5D+=+-4+%2B+5%2F.6065
(2) r%5Bs%5D+=+-4+%2B+8.244
(2) r%5Bs%5D+=+4.244 mi/hr answer
check answer:
(1) 5+=+%28r%5Bs%5D+-+4%29%2A%28t%5Bd%5D+%2B+20%29
(1) 5+=+%284.244+-+4%29%2A%28.6065+%2B+20%29
(1) 5+=+.244%2A20.6065
(1) 5+=+5.027 (rounding error?)
and
(2) 5+=+%28r%5Bs%5D+%2B+4%29%2At%5Bd%5D
(2) 5+=+%284.244+%2B+4%29%2A.6065
(2) 5+=+8.244%2A.6065
(2) 5+=+5