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A beam balance has two pans and a bar between them. Put marbles in a pan, and that pan tips down.

x + 7 = 12, and the beam lies flat.

Your textbook calls this Part II, Chapter 7, Finding the Unknown.

What you will learn

A balance — an equals sign says both sides match.
Expression or equation — only an equation has an equals sign.
Is it the one? — test a number in both sides.
Undo an add — take the same amount from both sides.
Undo a multiplication — share both sides the same way.
Two steps — undo the last step first.
From words — turn a bill into one short line.
Check it back — put the answer in the first line.
Both sides — lift the same letters from each pan.
A bracket — a group, as in 4(s + 3). Share both sides, or multiply each part.
No number fits — cancel the letters and read what is left.
The first slip — find the line where the answer changes.

Each lesson has a small lab. Tap it and try.