Nine sparrows land on a wire that already had some birds on it, and now there are 17 birds. Taking b as the number of birds before, which value of b makes b + 9 = 17 true?

Add an equals sign and you have an **equation** — a question. 'For what value of $x$ does $2x+3$ equal 13?' is written $2x+3=13$. The **solution** is the value that makes the two sides equal. Here, $x=5$ works, because $2\times 5+3=13$. For Class 6, the simplest method is **trial-and-improvement** — try whole numbers until you find one that works. Faster algebraic methods come in Class 7.