Finding the unknown number

This lesson asks you to use symbols as variables in expressions and equations, and to solve equations with whole numbers up to 50 and check the solutions. The museum offers the history of one early book on solving for an unknown; the equations and the checking are your class's. ¹

Write a problem as an equation with a symbol for the unknown. Solve it, then put your answer back into the equation to see whether both sides are equal.

A copy of al-Khwarizmi's book on algebra survives in the Bodleian Library at Oxford, written in Cairo ². The museum's record shows two pages of such a manuscript, with the proofs drawn as squares in red ink ³. Al-Khwarizmi wrote the book for what people need, in inheritance, bequests, trade and land measurement ⁴.

Al-Khwarizmi drew his proofs as squares in red ink, as well as writing his rules in words. When you check an answer, which helps you more, the words of the rule or a drawing, and why?

• Algebra: the book, who wrote it, and what it was for.
• The Book of al-Jabr and al-Muqabala: how the book is arranged, chapter by chapter.
• al-Khwarizmi: the scholar who wrote it.

Read the record Algebra. Make up a story problem with an unknown number up to 50, write it as an equation, solve it and check it.