Cogito
Grade 7 Math · Chapter 9 · Lesson 3
Probability as a Number
Nought means never, one means always.
10 problems · about 18 minutes · 7.SP.C.5
What this lesson teaches
The child expresses a probability as a number from 0 to 1 and interprets its size.
- A probability is a number from 0 to 1.
- It is the favourable outcomes divided by all the equally likely outcomes.
- All the outcomes together always add to 1.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
3 problemsReview — Samples and What They Tell You: 9 of 30 sampled. In 300, about how many?
Answer 90
Why About 90.
Review — Probability as a Number: A fair die. Out of 6, how many outcomes are less than 3?
Answer 2
Why 2 out of 6.
Review — When Three Measurements Fix a Triangle: Can sides 4, 4 and 9 make a triangle? 1 for yes, 0 for no.
Answer 0
Why No — 8 is less than 9.
Warm Up
Straightforward practice. Get the method working first.
3 problemsA die. How many outcomes give a number above 4?
Answer 2
Why 2 out of 6.
A die. How many outcomes give an even number?
Answer 3
Why 2, 4 and 6.
A die. How many outcomes in total?
Answer 6
Why Six faces.
Build It Up
The same ideas with more to keep track of.
2 problemsWhat is the probability of a certain event?
Answer 1
Why The top of the scale.
4 green out of 10. How many are not green?
Answer 6
Why 10 − 4.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Bag: 2 red and 8 blue. Out of 10, how many outcomes are red?
Answer 2
Why 2 out of 10.
The Certainty: What is the probability of an impossible event?
Answer 0
Why 0.