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Tiles are placed in a plus shape that grows each step. Can you find step 40 without building steps 1 to 39?

This is Part I, Chapter 4, Expressions using Letter-Numbers.

What you will learn

A letter-number is a letter that stands for a number.

• A letter for a number. n holds a number, not yet chosen.
• A rule in letters. One line answers every step.
• What 3n means. The times sign is dropped, nothing else.
• Putting a number in. Swap the letter for a value.
• Always, or only sometimes. One match is not proof.
• Terms of the same kind. Terms are parts of an expression. Sort by letter part.
• Adding whole expressions. Write all terms in one row. Join the like ones.
• From a situation. A fare or a bill, in one line.
• A formula. A rule in letters, like a polygon's perimeter.
• Letters in a calendar. One letter names every date in a square.
• Spot the slip. Find a small mistake by putting a number in.

Every teaching lesson has a lab. Set a number. Watch the line answer.

Unsure about patterns or the rectangle rule? Open those lessons first.