The AND and OR rules
The Maths Habit
You flip a coin and roll a dice.
Which is more likely:
a head, or a head and a six?
A head: needing a six as well makes it less likely
- AND
- Both things happen. Multiply the probabilities.
- OR
- One or the other happens. Add the probabilities.
A coin is flipped and a dice is rolled. The grid shows every outcome. What fraction of them are a head and a six?
1 out of 12 outcomes: 112
Half of the outcomes are heads. A six is what fraction of the heads, and so of the whole grid?
16 of 12 = 12 × 16 = 112
AND: multiply the probabilities
A dice is rolled. What fraction of the outcomes are a 3 or a 4?
16 + 16 = 26
OR: add the probabilities
Multiply or add?
Multiply or add?
Two fair coins are flipped. Find the probability of two heads.
A fair dice is rolled. Find the probability of a 3 or a 4.
Two fair coins are flipped. Find the probability of two heads.
A spinner lands on red with probability 13. It is spun twice. Find the probability of red both times.
Two fair coins are flipped. Find the probability of two heads.
A counter is taken from a bag. P(red) = 29 and P(blue) = 49. Find P(red or blue).
Two fair coins are flipped. Find the probability of two heads.
Two spinners are spun. P(A shows 5) = 14 and P(B shows 5) = 23. Find P(both show 5).
Two fair coins are flipped. Find the probability of two heads.
Each day Leo walks, cycles or takes the bus to school. P(walks) = 310 and P(cycles) = 110. Find P(walks or cycles).
Answer sheet
A fair coin is flipped three times. Find the probability of three heads.
18The probability of rain on any day is 13. Find the probability it rains on Saturday but not on Sunday.
29
P(bus late) = 13, P(train late) = 25. Find P(both late).
215A spinner: P(red) = 0.3, P(blue) = 0.45. Find P(red or blue).
0.75Ava says P(head and a 6) = 12 + 16 = 23. Is Ava right?
No: multiply, 112