The Student Room Group
Uni students - Complete our survey >>
Uni students - Complete our survey >>
Uni students - Complete our survey >>

Stuck on question (Probability or possible quadratic equation)

Hi :smile:.

Not sure how to do this question. It mentioned probability but am I to just solve it like a quadratic equation?

Thanks
Screen Shot 2016-05-16 at 17.15.34.png29.9KB
Reply 1
Avatar for Zacken
Zacken
22
Original post by jojo55
Hi :smile:.

Not sure how to do this question. It mentioned probability but am I to just solve it like a quadratic equation?

Thanks


No. Draw a tree diagram. What is the probability that Joshua takes one white sock and one black sock in terms of y?

Equate this expression to 6/11 and simplify/re-arrange to get the given quadratic equation. They do not want you to solve it just yet, they want you to use the facts given to get that quadratic.
Reply 2
Avatar for jojo55
jojo55
OP
11
Original post by Zacken
No. Draw a tree diagram. What is the probability that Joshua takes one white sock and one black sock in terms of y?

Equate this expression to 6/11 and simplify/re-arrange to get the given quadratic equation. They do not want you to solve it just yet, they want you to use the facts given to get that quadratic.




I get yy+5\frac{y}{y+5} to show the probability of a white sock out of the total amount of socks of y + 5

Plus, I get 5y+5\frac{5}{y+5} to show the probability of a black sock.

Is the next bit:

(yy+5\frac{y}{y+5})(5y+5\frac{5}{y+5}) = 611\frac{6}{11}

Not 100% on that as the mark scheme says it is 2(yy+5\frac{y}{y+5})(5y+5\frac{5}{y+5}) = 611\frac{6}{11} instead.
Reply 3
Avatar for Zacken
Zacken
22
Original post by jojo55
I get yy+5\frac{y}{y+5} to show the probability of a white sock out of the total amount of socks of y + 5

Plus, I get 5y+5\frac{5}{y+5} to show the probability of a black sock.

Is the next bit:

(yy+5\frac{y}{y+5})(5y+5\frac{5}{y+5}) = 611\frac{6}{11}

Not 100% on that as the mark scheme says it is 2(yy+5\frac{y}{y+5})(5y+5\frac{5}{y+5}) = 611\frac{6}{11} instead.


You've done:

P(white sock) * P(black sock).

What about the other possibility of:

P(black sock) * P(white sock)?

You need to add those together:

P(white sock)*P(black sock) + P(black sock)*P(white sock) = yy+5⋅5y+5+5y+5⋅yy+5\frac{y}{y+5} \cdot \frac{5}{y+5} + \frac{5}{y+5} \cdot \frac{y}{y+5}

Did you draw a tree diagram? If so, it should have been obvious of the two different routes you can go to get the requires probability.
Original post by jojo55
Hi :smile:.

Not sure how to do this question. It mentioned probability but am I to just solve it like a quadratic equation?

Thanks


took me a while but i think i got it lol:

2(y/y+5 x 5/y+4) = 6/11

2(5y/y2 + 9y + 20) = 6/11

10y/y2 + 9y + 20 = 6/11

110y/y2 + 9y + 20 = 6

110y = 6y2 + 54y + 120

0= 6y2 - 56y + 120

0= 3y2 -28y +60
Reply 5
Avatar for Zacken
Zacken
22
Original post by qwerty0301
took me a while but i think i got it lol:


Full solutions are against forum guidelines, have you read the posting guide? :smile:

Thanks. :smile:
Reply 6
Avatar for jojo55
jojo55
OP
11
Original post by Zacken
Full solutions are against forum guidelines, have you read the posting guide? :smile:

Thanks. :smile:


Original post by qwerty0301
took me a while but i think i got it lol:

2(y/y+5 x 5/y+4) = 6/11

2(5y/y2 + 9y + 20) = 6/11

10y/y2 + 9y + 20 = 6/11

110y/y2 + 9y + 20 = 6

110y = 6y2 + 54y + 120

0= 6y2 - 56y + 120

0= 3y2 -28y +60



Thanks for the help.

I got the same solution after I figured out that that it wasn't both over y+5, that I put in my earlier post.

Thanks again.

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you