In this activity, we'll be learning how to change missing number problems into algebra.

 

So first... what is algebra?

 

 

We use algebra when we don't know the exact numbers in a calculation, and to do this we use letters to stand for numbers that we are trying to find.

 

 

Algebra is like a language with its own rules!  Let's look at some of the rules we need to know.

 

1) a stands for a number (we can use any letter)

2) a + 2 is 2 more than the number.

3) a - 7 is 7 less than the number.

4) 5a is 5 times the number (we don't use the × sign in algebra because it looks like the letter x).

5) a/4 or ¼ a is a quarter of the number or the number divided by 4. (N.B. We don't use the ÷ sign in algebra.)

 

 

Let's now use our knowledge to work out the following problem:

 I think of a number.

1)  I double it.

2) Take away 5.

3) I get 21.

 

Write this using algebra.

Use n for the number I think of.

 

Solution

n is the number I think of.

1) Double it to get 2n (remember this means 2 times n)

2) Take away 5 to get 2n - 5

3) This equals 21, so we write:

2n - 5 = 21

 

Do you get it now?

 

 

Let's have a go at some questions then!