The steps of a temple are being tiled so that step number $n$ needs $8n+5$ tiles. How many tiles does step 12 need?

Imagine you want to describe 'a number doubled, plus three'. Without algebra you would have to make a list: $2\times 1+3=5$; $2\times 2+3=7$; $2\times 5+3=13$; … forever. With a letter — say $x$ for 'the number' — the whole list becomes one short expression: **$2x+3$**. Pick any value of $x$ and you get the right answer. The letter is shorthand for 'whatever number we're talking about'.