Question:

Check my answer on probability~?

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A ball is drawn from a box containing 10 red, 15 black, 5 green and 10 yellow balls. Find the probability that the ball is

(a) not green or yellow

My solutions:

P(G U Y) = P(G) + P(Y) = 1/8 + 1/4 = 3/8

1 - 3/8 = 5/8.

[I'm confused here: the formula is P(A U B) = P(A) + P(B) - P(A and B)... So I should find P(G and Y) too right...*blur*]

(b) not yellow

1 - 30/40 = 1 - 3/4 = 1/4

(c) red or black or green

My solutions:

10/40 + 15/40 + 5/40

= 1/4 + 3/8 + 1/8

= 2/8 + 3/8 + 1/8

= 6/8

= 3/4

[For this, are my workings correct? I know my answer is correct, if my workings are incorrect please do correct me..]

(d) not blue

My solutions:

[no idea]

Hmm I have to use this formula in some parts of the question right...

P(A U B) = P(A) + P(B) - P(A and B)

But I only use the P(A) + P(B)... eh.. I'm confused...

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5 ANSWERS


  1. a) is right

    b) should be 3/4  1-(1/4)

    c) is right

    d) i don't see blue in the problem so i'm guessing the probability should be 1.  all i see is yellow, green, black, and red


  2. You did right in (a), (b), and (c)

    (d) The probability = 1 since none of the balls is blue.

  3. (a) not green or yellow:

    = 100%(25/40)

    = 100%(5/8)

    = 500/8% or 62.5%

    Answer: 5/8 or 62.5%: You're right!

    (b) not yellow:

    = 100%(30/40)

    = 100%(3/4)

    = 300/4% or 75%

    Answer: 3/4 or 75%

    (c) red or black or green:

    = 100%(30/40)

    = 3/4 or 75%

    Answer: 3/4 or 75%: it's the same as saying "not yellow". You're right!

    (d) not blue:

    Answer: 100% because there isn't any blue ball in the box.

  4. Hmm I have to use this formula in some parts of the question right...

    P(A U B) = P(A) + P(B) - P(A and B)

    But I only use the P(A) + P(B)... eh.. I'm confused...

    In your problems, all P( A and B ) are Zero.

    Each ball have only one color, you can not draw a

    ball with two or more colors.

  5. For this problem, there is no need for A,B etc., unless you do want to use them.

    There are 40 balls in a box.

    10 red, 15 black, 5 green and 10 yellow

    a) not green or yellow

    count the red and black.

    25/40 = 5/8

    not yellow: 30/40 =3/4

    c) correct

    d) not blue is 1. There are no blue balls, so the probability of a ball not being blue is a certainty.

    a) b) and c) are correct.

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