Suleika has six cards numbered 1 to 6.
(a) She takes one card at random, records the number and replaces the card.
(i) Write down the probability that the number is 5 or 6.
(ii) Suleika does this 300 times.
Find how many times she expects the number 5 or 6.
(b) Suleika takes two cards at random, without replacement.
(i) Find the probability that the sum of the numbers on the two cards is 5.
(ii) Find the probability that at least one of the numbers on the cards is a square number.
▶️ Answer/Explanation
(a)(i) Ans: \(\frac{1}{3}\)
There are 2 favorable outcomes (5 or 6) out of 6 possible cards. Thus, the probability is \(\frac{2}{6} = \frac{1}{3}\).
(a)(ii) Ans: 100
Multiply the probability \(\frac{1}{3}\) by the number of trials (300): \(\frac{1}{3} \times 300 = 100\).
(b)(i) Ans: \(\frac{2}{15}\)
Possible pairs summing to 5: (1,4), (2,3), (4,1), (3,2). There are 4 favorable outcomes out of \(6 \times 5 = 30\) total outcomes, giving \(\frac{4}{30} = \frac{2}{15}\).
(b)(ii) Ans: \(\frac{3}{5}\)
Square numbers in 1-6: 1, 4. Probability of neither card being square is \(\frac{4}{6} \times \frac{3}{5} = \frac{2}{5}\). Thus, the probability of at least one square is \(1 – \frac{2}{5} = \frac{3}{5}\).
Ravi spins a biased 5-sided spinner, numbered 1 to 5.
The probability of each number is shown in the table.
(a) Find the value of x.
(b) Ravi spins the spinner once.
Find the probability that the number is 2 or 3.
(c) Ravi spins the spinner twice.
Find the probability that
(i) the number is 2 both times,
(ii) the sum of the numbers is 3.
(d) Ravi spins the spinner 72 times.
Calculate how many times he expects the number 1.
▶️ Answer/Explanation
(a) Ans: \(\frac{1}{8}\)
Since probabilities sum to 1, \(x = 1 – \left(\frac{1}{6} + \frac{1}{4} + \frac{1}{3} + \frac{1}{8}\right) = \frac{1}{8}\).
(b) Ans: \(\frac{7}{12}\)
P(2 or 3) = P(2) + P(3) = \(\frac{1}{4} + \frac{1}{3} = \frac{7}{12}\).
(c) (i) Ans: \(\frac{1}{16}\)
P(2 both times) = \(\left(\frac{1}{4}\right)^2 = \frac{1}{16}\).
(c) (ii) Ans: \(\frac{2}{24}\) or \(\frac{1}{12}\)
Sum is 3 only for (1,2) or (2,1). P = \(\left(\frac{1}{6} \times \frac{1}{4}\right) + \left(\frac{1}{4} \times \frac{1}{6}\right) = \frac{2}{24}\).
(d) Ans: 12
Expected count for number 1 = \(72 \times \frac{1}{6} = 12\).