Basic Probability
1. Problem: Find the probability of getting an even number when a die is rolled.
The possible even numbers on a die are 2, 4, and 6. So, there are 3 favorable outcomes.
Total outcomes = 6.
Probability = number of favorable outcomes / total outcomes = $\frac{3}{6} = \frac{1}{2}$.
2. Problem: Find the probability that the sum of two dice rolled is 7.
Possible pairs that sum to 7: (1,6), (2,5), (3,4), (4,3), (5,2), (6,1) = 6 pairs.
Total outcomes = $6 \times 6 = 36$.
Probability = $\frac{6}{36} = \frac{1}{6}$.
3. Problem: Find the probability of getting heads when a fair coin is tossed once.
There are 2 possible outcomes: Heads and Tails.
Probability = $\frac{1}{2}$.
4. Problem: The probability of rain tomorrow is 0.3. Find the probability it will not rain.
Using complementary probability: Probability(not rain) = $1 - 0.3 = 0.7$.
5. Problem: A bag contains 3 red balls and 2 blue balls. Find the probability that a randomly drawn ball is not red.
Total balls = 3 + 2 = 5.
Number of non-red balls = 2.
Probability = $\frac{2}{5}$.
6. Problem: A card is drawn from a standard deck. Probability it is not a face card.
Face cards = Jack, Queen, King in each of 4 suits = 12 cards.
Total cards = 52.
Probability(not face) = $\frac{52 - 12}{52} = \frac{40}{52} = \frac{10}{13}$.
7. Problem: Lottery ticket costs 1, prize is 10 with probability 0.1. Find expected value.
Expected winning = $10 \times 0.1 = 1$.
Expected cost = 1.
Expected value = Expected winning - cost = $1 - 1 = 0$.
8. Problem: A player pays 5 to play; wins 30 if rolls 6, else wins 0. Find expected net gain/loss.
Probability(roll 6) = $\frac{1}{6}$.
Expected winnings = $30 \times \frac{1}{6} + 0 \times \frac{5}{6} = 5$.
Net gain/loss = Expected winnings - cost = $5 - 5 = 0$.
9. Problem: Probability of winning is 0.25. Find odds in favor.
Odds in favor = $\frac{P(Win)}{1 - P(Win)} = \frac{0.25}{0.75} = \frac{1}{3}$.
10. Problem: Odds in favor of drawing a heart from a deck.
Number of hearts = 13.
Number of non-hearts = 52 - 13 = 39.
Odds in favor = $13:39 = 1:3$.
11. Problem: Odds against rolling a prime number on a die.
Prime numbers on a die: 2,3,5 = 3.
Non-prime = 6 - 3 = 3.
Odds against = number of unfavorable : favorable = $3:3 = 1:1$.
12. Problem: Odds in favor of drawing two black balls in succession without replacement from 7 white and 5 black balls.
Total balls = 12.
First black ball probability = $\frac{5}{12}$.
After drawing one black, remaining black = 4, total = 11.
Second black probability = $\frac{4}{11}$.
Probability both black = $\frac{5}{12} \times \frac{4}{11} = \frac{20}{132} = \frac{5}{33}$.
Probability not both black = $1 - \frac{5}{33} = \frac{28}{33}$.
Odds in favor = $\frac{5}{33} : \frac{28}{33} = 5 : 28$.
13. Problem: Probability of getting either 2 or 5 on a die.
Favorable outcomes = 2 (either 2 or 5).
Total = 6.
Probability = $\frac{2}{6} = \frac{1}{3}$.
14. Problem: Probability of drawing either a heart or a king from 52 cards.
Hearts = 13, Kings = 4.
King of hearts counted twice, so subtract 1.
Total favorable = $13 + 4 - 1 = 16$.
Probability = $\frac{16}{52} = \frac{4}{13}$.
15. Problem: Probability a student takes Mathematics or Physics given 60% take Mathematics, 45% take Physics, 25% take both.
Use formula: $P(M \cup P) = P(M) + P(P) - P(M \cap P)$.
$= 0.6 + 0.45 - 0.25 = 0.8$.
Final answers:
1. $\frac{1}{2}$
2. $\frac{1}{6}$
3. $\frac{1}{2}$
4. 0.7
5. $\frac{2}{5}$
6. $\frac{10}{13}$
7. 0
8. 0
9. $\frac{1}{3}$
10. $1:3$
11. $1:1$
12. $5:28$
13. $\frac{1}{3}$
14. $\frac{4}{13}$
15. 0.8