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Word problem on finding speed based on time, distance

  1. Aug 17, 2015 #1
    Mod note: Moved the text under Relevant equations to the attempt section
    1. The problem statement, all variables and given/known data

    Hello!
    Can't wrap my head around this easy problem. Would be grateful for help in spotting out my mistakes.
    One day, Donnie observes that the wind is blowing at 6 miles per hour. An unladen swallow
    nesting near Donnie's house flies three quarters of a mile down the road (in the direction of
    the wind), turns around, and returns exactly 4 minutes later. What is the airspeed of the
    unladen swallow? (Here, `airspeed' is the speed that the swallow can
    y in still air.)
    2. Relevant equations


    3. The attempt at a solution
    Here is how I approached the solution:
    x - airspeed in miles/ hour
    v1 - speed of the swallow in the direction of the wind, in miles/hour
    v2 - speed on the way back in miles/ hour
    v1 = x + 6
    v2 = x - 6
    time to fly 3/4 of a mile = 3/4 : v1
    time to fly back = 3/4 : v2
    overall time = 1/15 (4 minutes)

    then, overall time: 1/15 = 3/4 : (x+6) + 3/4 : (x-6)

    When I try to solve this, I do not get the result any way near the desired one, and I do not get correct roots for x.
     
    Last edited by a moderator: Aug 17, 2015
  2. jcsd
  3. Aug 17, 2015 #2
    Check that calculation once again .
     
  4. Aug 17, 2015 #3
    I did quite a few times :)
    the answer should be 24 miles/hour

    15*3(x-6) + 15*3(x+6) - 4(x^2 - 36) = 0
    -4x^2 + 90x + 144 = 0
    doesn't give the 24m/h
     
  5. Aug 17, 2015 #4
    Okay - show your working of the quadratic equation's solution . It's correct till now .
     
  6. Aug 17, 2015 #5
    -90 +- √90*90 +4*4*144 / -16
    -90 - 102 / - 16 = 12
     
  7. Aug 17, 2015 #6
    Two things - First - You should be using brackets appropriately .
    Second - What's the formula used to solve a quadratic ? Does the denominator use 4*a or 2*a ?
     
  8. Aug 17, 2015 #7
     
  9. Aug 17, 2015 #8
    exactly! thank you, and sorry for not posting correct brackets. of course, it is 2a in the denominator :) Thank you!
     
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