QN's answer to QN's Junior College 1 H1 Maths Singapore question.
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I didn’t understand their method, is it wrong?
Date Posted:
4 years ago
Erm my answer was for iii), not iv)
Here's my working for iv) :
P(at least 2 red socks out of 3 now)
= P(all 3 red)
+ P(2 red, 1 other colour)
= P(all 3 red)
+ P(1st red, 2nd not red, 3rd red)
+ P(1st red, 2nd red, 3rd not red)
+ P(1st not red, 2nd red, 3rd red)
= 10/25 x 9/24 x 8/23
+ 10/25 x 15/24 x 9/23
+ 10/25 x 9/24 x 15/23
+ 15/25 x 10/24 x 9/23
= (10x9x8 + 3x10x15x9)/(25 x 24 x 23)
= 4770/13800
= 477/1380
= 159/460
P(at least 2 red socks out of 3 now)
= P(all 3 red)
+ P(2 red, 1 other colour)
= P(all 3 red)
+ P(1st red, 2nd not red, 3rd red)
+ P(1st red, 2nd red, 3rd not red)
+ P(1st not red, 2nd red, 3rd red)
= 10/25 x 9/24 x 8/23
+ 10/25 x 15/24 x 9/23
+ 10/25 x 9/24 x 15/23
+ 15/25 x 10/24 x 9/23
= (10x9x8 + 3x10x15x9)/(25 x 24 x 23)
= 4770/13800
= 477/1380
= 159/460
My bad I send the wrong part
Even for part(iv),
Shouldn’t it be
P(all 3 red)
+ P(1st red x 2nd red x 3rd pink)
+ P(1st red x 2nd red x 3rd blue)
+ P(1st pink x 2nd red x 3rd red)
+ P(1st blue x 2nd red x 3rd red)
Shouldn’t it be
P(all 3 red)
+ P(1st red x 2nd red x 3rd pink)
+ P(1st red x 2nd red x 3rd blue)
+ P(1st pink x 2nd red x 3rd red)
+ P(1st blue x 2nd red x 3rd red)
At least 2, and there are different probabilities when I first pick red, blue or pink socks
Yes, but realise something : you've missed out these probabilities.
P(1st red, 2nd blue, 3rd red)
P(1st red, 2nd pink, 3rd red)
P(1st red, 2nd blue, 3rd red)
P(1st red, 2nd pink, 3rd red)
Ah thank you
What I did in my working was , instead of separately computing the cases with ① 2 reds 1 pink and ②2 reds 1 blue,
I choose to combine them together and do the various orders of 2 red, 1 non-red
(Non-red already covers both pink and blue)
You can try to do for the separate case and you should still get 159/460
I choose to combine them together and do the various orders of 2 red, 1 non-red
(Non-red already covers both pink and blue)
You can try to do for the separate case and you should still get 159/460
Edited
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