probability?

對有1分，錯扣1分，冇答冇分，答對機率0.7, 答錯機率0.2, 冇答機率 = 1 - 0.7 - 0.2 = 0.1。

26a)
P(2 marks) = P(對,對) = 0.7 * 0.7 = 0.49

26b)
P(1 mark) = P(對,冇答) + P(冇答,對) = 0.7 * 0.1 + 0.1 * 0.7 = 0.14

26c)
P(0 mark) = P(對,錯) + P(錯,對) + P(冇答,冇答) = 0.7 * 0.2 + 0.2 * 0.7 + 0.1 * 0.1 = 0.29

26d)
P(- 1 mark) = P(錯,冇答) + P(冇答,錯) = 0.2 * 0.1 + 0.1 * 0.2 = 0.04

26e)
P(- 2 marks) = 1 - 0.49 - 0.14 - 0.29 - 0.04 = 0.04
或 = P(錯,錯) = 0.2 * 0.2 = 0.04