In a letter simply stands for a number we don't know yet. Once we have a letter, we can write a short phrase in plain words as a tidy expression — a little recipe made of numbers, letters and operations.

Read each phrase slowly and ask: what is happening to the unknown number? If we call the unknown number n, then:

• “5 more than n” means add five: n + 5
• “double a” means two of them: 2a
• “3 less than x” means take three away: x - 3
• “p lots of q” means multiply: pq

Notice that 2a and pq drop the \times sign — in algebra, writing two things side by side already means multiply. (When several operations meet, the usual still applies.)

Watch a phrase turn into its expression. The words appear first; then the matching algebra grows underneath. The letter is just a stand-in for a number we haven't been told yet.

Khan Academy walks through writing expressions from words here: