Random events and probability
Answers are for parents and teachers.
1
Cards are numbered 1 to 12. Find the probability that a randomly drawn card shows a multiple of 3.
3, 6, 9, 12 — 4 cards: 4/12 = 1/3.
2
A basket holds 7 apples, 5 pears and 8 plums. What is the probability that a randomly chosen fruit is not a pear?
There are 20 in total; 15 are not pears: 15/20 = 3/4.
3
Find the probability that two tossed dice sum to 10.
(4, 6), (5, 5), (6, 4) — 3 pairs, 3/36 = 1/12.
4
A test has 4 yes/no questions and a student marks the answers at random. Find the probability that all answers are correct and the probability that at least one is correct.
There are 2⁴ = 16 equally likely outcomes. All correct: 1/16; at least one correct: 16 − 1 = 15 outcomes, 15/16.
5
How do you understand the forecast “the probability of rain tomorrow is 30 %”?
In about 30 % of days with similar conditions it has rained. It is more likely not to rain tomorrow, but rain is still possible.