simple math question

[missy77]

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Hello guys:

I have a question on probability; It's kinda basic. Here we go:

A)Three coins are tossed. Fine the probability of getting atleast two tails. I know how to solve this. The event of at least two tails means two tails and one head, or three tails. There are four outcomes: TTH,THT,HTT, TTT= 4/8 =1/2
I totally understand this but the real problem is on the next question:
B)A box contains 3 orange balls, 3 yellow balls, and 2 white balls. Three balls are selected without replacement. Find the probability of selecting 2 yellow balls and a white ball.
answer: 3/8 x 2/7x2/6= 1/28 why here we don't say(like we did for coins)
white-yellow-yellow= 1/28 or yellow-white-yellow or yellow-yellow-white
so 3 x 1/28 = 1/7 ?

thanks

 

[americanpierg]

Hello guys:

I have a question on probability; It's kinda basic. Here we go:

A)Three coins are tossed. Fine the probability of getting atleast two tails. I know how to solve this. The event of at least two tails means two tails and one head, or three tails. There are four outcomes: TTH,THT,HTT, TTT= 4/8 =1/2
I totally understand this but the real problem is on the next question:
B)A box contains 3 orange balls, 3 yellow balls, and 2 white balls. Three balls are selected without replacement. Find the probability of selecting 2 yellow balls and a white ball.
answer: 3/8 x 2/7x2/6= 1/28 why here we don't say(like we did for coins)
white-yellow-yellow= 1/28 or yellow-white-yellow or yellow-yellow-white
so 3 x 1/28 = 1/7 ?

thanks

(3C2 * 2C1) / 8C3 = 3 * 2 / 56 = 6/56 = 3/28

3/28 doesn't equal 1/7. Your reasoning does work. 1/28 ways to selecting 2 yellows and 1 white, and 3 different ways to arrange those 3. 1/28*3= 3/28

These problems are easier and faster when you learn how to use equations as shortcuts (while still understanding why the equations work). Thinking them out logically and writing all the combinations out will get really hard when you are given large combinations. Say it said probability of selecting 2 yellow balls, a white ball, and 2 orange balls. Finding the chances of getting 2, 1, and 2 will be simple, but finding how those 5 will arrange by writing them all out will take an extra 1-2 minutes, time u can't afford to waste on this section.

 

[wantVCUdental]

Both of you are correct and Americanperg raises a good point. but you both got the same answer
to the Open poster, i don't know where you got the question from, i believe the answer is incorrect and that we are correct and the answer should be 3/28 not 1/28

and no 3/28 not= 1/7 😀

 

[missy77]

Thanks for your help; Yeah sorry it was late around 2 am 😛 For some reason I thought 3/21. Hopefully during the actual test I won't make such a mistake.