Physics average speed qusetion pls help :)?

Your average speed on the first half of a car trip is 74.0 km/h. How fast do you have to drive on the second half of the trip to achieve an average speed of 85.0 km/h for the whole trip?

can someone pls explain this oen to me?

thankss youuss :)!

1 Answer

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  • EM
    Lv 7
    10 years ago
    Favorite Answer

    distance = (rate)(time)

    For the first half of the trip:

    x/2 = (74.0 km/h)(t1)

    t1 = x/(2 * 74.0 km/h) [1]

    For the second half:

    x/2 = (v)(t2)

    t2 = x/(2v) [2]

    For the whole trip:

    x = (85.0 km/h)(t1 + t2) [3]

    Adding [1] & [2], substitute into [3], and solve for v:

    t1 + t2 = x/(2 * 74.0 km/h) + x/(2v)

    x = (85.0 km/h)(x/(2 * 74.0 km/h) + x/(2v))

    1 = (85.0 km/h)(1/(2 * 74.0 km/h) + 1/(2v))

    1/(85.0 km/h) - 1/(2 * 74.0 km/h) = 1/(2v)

    (85.0 km/h)(74.0 km/h) / [(2 * 74.0 km/h) - (85.0 km/h)] = v

    v = 99.8 km/h

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