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.

Now your turn on the balance. First we sort out one new kind of sentence together, then you slide and Check four of your own.

Look at this sentence: 12 = a + 5. The letter is on the right this time. 12 is the whole on the left. On the right, a is one part and 5 is the other part. The missing part is what makes both sides equal, so a is 7 because 7 + 5 = 12.

Now the four challenges. Slide the letter and press Check. First a straight add: b + 7 = 15. Then the letter on the right, just like we did: 16 = c + 9. Then the letter is the whole made from the two parts on the right: e = 3 + 6. Last, the letter is the whole again, but from different parts: f = 4 + 3.