f5 maths

two fair dice are thrown .find the probability that the sum of two numbers is
(a) less than 4
(b) not equal to 5
(c)4,5 or12
(a)
The satisfy combination
(1,1) (1,2) (2,1)
the probability
=3/36
=1/12
(b)
The not satisfy combination
(1,4) (2,3) (3,2) (4,1)
the probability
=1-4/36
=8/9
(c)
The satisfy combination
(1,3) (2,2) (3,1) (1,4) (2,3) (3,2) (4,1) (6,6)
the probability
=8/36
=2/9

probability係好容易答的題,但係要好少心,一定要知道會有幾多變數否則就會成題錯晒!
首先two dice 共有36個變數係1,1&1,2&1,3&1,4&1,5&1,6&2,1............6,6
a:.less than 4 只有1,1&1,2&2,1 即是有3 個可能 so the answer is 3/36=1/12(12份1)
b: equal to 5只有1,4&2,3&3,2&4,1即是只有4個可能,那麼不等於5的機會即是36-4=32,就係32/36=8/9
c:.equal to 4 有1,3&2,2&3,1有3個可能=3/36,equal to 有4個可能(參考b)=4/36,equal to 12 有6,6 即是1/36.答案是3/36+4/36+1/36=8/36=1/4.