LEARN · EXPLAIN · REVISE

Choose one book: t mathematics titles or 3 history titles, with no overlap. How many choices?

Read the idea, work independently, then explain what changed.

TOPIC 01

Counting principles: mixed review

A mixed assessment: identify the method, justify it and revise your reasoning.

Use one hint at a time. A correction explains what changed, not just the final answer.

01 / Foundation#Your turn

t=20

Addition requires disjoint cases; stage counts must hold for each branch or use separate branches.

Skills and prerequisite lessons

Working and explanation

BUILD THE REASONING

Hint 1

Decide whether choices are alternative cases or successive stages, then check overlap.

Hint 2

Use this intermediate relation.

N=t+3

Worked solution

Decide whether choices are alternative cases or successive stages, then check overlap.
Apply the stated relation and retain its conditions.
$N=20+3=23$
The two subject cases are disjoint alternatives.

The requested value is 23.

Checks and common pitfalls: The two subject cases are disjoint alternatives.

Think first. Reveal a hint when the class is ready.

Teacher preparation and assessment