probability 2

(a). required outcome: (1+9) & (3+7)
all the possible outcome: 5C2 = 10
required probability: 2/10 = 1/5

(b). required probability = 1

(as the sum of any 2 odd numbers must be an even number, thats all outcome should be fitted in this case)

(c). required outcome: (1,5), (3,9), (5,7)
all the possible outcome: 5C2 = 10
required probability: 3/10

the answer is B!!
Why??
Answer:
1+3=4 1+5=6 1+7=8 1+9=10
3+1=4 3+5=8 3+7=10 3+9=12
5+1=6 5+5=10 5+7=12 5+9=14
7+1=8 7+5=12 7+7=14 7+9=16
9+1=10 9+5=14 9+7=16 9+9=18

A box contains five balls numbered with 1, 3, 5, 7, and 9 separately. Two balls are drawn from the box one by one without replacement. Find the probability that the sum of the numbers on the balls drawn is



(a). 10
1+9=10
7+3=10

只有這2種情況

P(1+9 or 9+1) = (1/5)*(1/4) * 2 = 1/10
P(7+3 or 3+7 ) = (1/5)*(1/4) * 2 = 1/10
P(1+9 or 9+1 or 7+3 or 3+7 ) = 1/10 * 2 = 1/5


(b). an even number

1+3=4
1+5=6
1+7=8
1+9=10

3+1=4
3+5=8
3+7=10
3+9=12

5+1=6
5+3=8
5+7=12
5+9=14

7+1=8
7+3=10
7+5=12
7+9=16

9+1=10
9+3=12
9+5=14
9+7=16

全部case 都可以變成even number~
所以P(sum is even number ) = 1

(c). divisible by 3

1+3=4
1+5=6---------divisible by 3
1+7=8
1+9=10

3+1=4
3+5=8
3+7=10
3+9=12---------divisible by 3

5+1=6---------divisible by 3
5+3=8
5+7=12---------divisible by 3
5+9=14

7+1=8
7+3=10
7+5=12---------divisible by 3
7+9=16

9+1=10
9+3=12---------divisible by 3
9+5=14
9+7=16

P---------(divisible by 3) = 6/20 = 3/10