SOLUTION: Question: Steve and Michael are in their boat headed upstream. They travel a distance of 15 miles to the nearest beach. The speed of the current is 4 miles per hour. The total trip

Algebra ->  Customizable Word Problem Solvers  -> Travel -> SOLUTION: Question: Steve and Michael are in their boat headed upstream. They travel a distance of 15 miles to the nearest beach. The speed of the current is 4 miles per hour. The total trip      Log On

Ad: Over 600 Algebra Word Problems at edhelper.com


    

Question 589633: Question: Steve and Michael are in their boat headed upstream. They travel a distance of 15 miles to the nearest beach. The speed of the current is 4 miles per hour. The total trip to the beach takes 7 hours. What is the speed of the boat?
My work: I thought I could try using a variation of t=d/r
I thought I could do (30/(x-4))+(30/(x+4))=7 where x=speed of the boat
Is this the correct way to solve the problem?
Answer by mananth(16946) About Me  (Show Source):
You can put this solution on YOUR website!
Boat speed x mph
current speed 4 mph

against current x- 4 mph
with current x+ 4 mph

Distance=15 miles

Time against + time with =7hours
t=d/r

15/(x+4) +15/(x-4)=7

LCD =(x-4)(x+4)
15*(x-4) +15(x+4) = 7
15x-60+15x+60=7(x ^2-16)
30x=7x ^2-112
7x ^2-30 x-112

Find the roots of the equation by quadratic formula

a= 7 , b= -30 , c= -112

b^2-4ac= 900 + 3136
b^2-4ac= 4036
%09sqrt%28%094036%09%29=%0963.53%09
x=%28-b%2B-sqrt%28b%5E2-4ac%29%29%2F%282a%29
x1=%28-b%2Bsqrt%28b%5E2-4ac%29%29%2F%282a%29
x1=( 30 + 63.53 )/ 14
x1= 6.68
x2=( 30 -63.53 ) / 14
x2= -2.39
Ignore negative value
Boat speed 6.68 mph

m.ananth@hotmail.ca