ACTIVITY 7

You can solve and complete number sentences by: • inspection • trial and improvement • checking the solution by substitution

Let us look at solving number sentences by inspection.

The inspection method is another name for ‘guess the answer’. If you look at a number sentence and must find the answer without making calculations, you will use inspection.

In maths, we only solve simple number sentences by inspection.

Example: Study the number sentence: 923 – 25 =

We are not going to draw any pictures to solve this number sentence. Use mental maths and decide on an answer.

You will then see – by using inspection – that the answer is 898.

1. Solve the number sentences using inspection.

Question

1.1 1 273 + 6 = 1.2 662 + 35 = 1.3 997 – 133 = 1.4 412 – 12 = 1.5 540 – 120 =

Answer 1 279 697 864 400 420

Now let us look at solving number sentences through trial and improvement.

Using this method, you will guess an answer, but you will write it down and then calculate the number sentence. If your answer is incorrect, you can try another answer and calculate the number sentence until it is correct or until the number sentence is TRUE.

This number sentence is FALSE:

2 413 – 12 = 2 410

This number sentence is TRUE:

2 413 – 12 = 2 401

Example: Study the number sentence below and use trial and improvement to solve it.