Here are two number sentences side by side:

□ + 3 = 9 and a + 3 = 9

One uses a box for the missing number. The other uses a letter. Is the missing number the same in both? Hands up: what do you think that missing number is?

Watch the balance beams. Each shows a number sentence with a letter for the unknown. Look for the value that levels the beam. The letter does the same job a box would do.

Now a fresh letter in a different sentence, b + 4 = 10.

The last one is different. Here the letter sits on its own on the left because the letter is the whole. On the right sit the two parts, 5 and 2. So d = 5 + 2. See if the same idea still works.

Let's work one out together on the balance. Our sentence is a + 4 = 9, written with a letter. We slide the letter until the beam hangs level. Then we say what the letter is standing in for. Remember: 9 is the whole and 4 is one part, so a has to be the other part.