ACTIVITY 8

Attempt 2: To choose 6 as the answer will make the number sentence FALSE. Guess another answer. Since 6 is not enough to subtract from 156 to get 149, you must choose a greater number. Use the number 7.

156 – 7 = 149 

Using this method, it may take a number of tries to solve the number sentence. It is time-consuming and not always the best method to use.

ACTIVITY 8

1. Solve the number sentences using the trial and improvement method.

Question

1.1 458 –  = 453 1.2 29 +  = 143 1.3 985 +  = 991

1.4 326 –  = 216 1.5 15 +  = 105

Answer 5 114

6 110

90

Checking the solution by substitution is very similar to the trial and improvement method. You have to study the number sentence and then decide on an answer. You then replace the answer in the number sentence and do the calculation. The number sentence must be TRUE.

Number sentences are also a way of showing equivalence. What does equivalence mean?

Equivalence is when one side of the number sentence is equal to the other side of the number sentence. In other words, showing that the number sentence is true.

The left side of the number sentence is equivalent (equal to) the right side of the number sentence. It means that the number sentence is true.

Number sentences may also be used in flow charts and tables. Look at a few examples.

In the flow chart below the number sentence is on the left-hand side and the equivalent value is on the right-hand side. Complete the flow chart so that all the number sentences have equivalent values.

4 + 256

156 – 7

19 + 199

86 + 359

999 – 90 260

149

218

445

909

Addition and subtraction in number sentences

You already know that addition and subtraction are opposites. If you add 6 sweets to the 4 you already have, you will have 10 sweets. To return to what you originally had, you are going to do the opposite. Instead of adding 6 sweets, you are going to subtract them. The 10 sweets minus the 6 sweets gives you the original 4 sweets you had to start with. If you write this in a number sentence, it will look like this:

Description in words

I have 4 sweets and add 6. I have 10 sweets and subtract 6.

Number sentence

4 + 6 = 10 10 – 6 = 4

Can you think of more examples where addition and subtraction can be used as opposite calculations in number sentences? Let the learners write down more of their own examples in the space provided in the study guide.

Learners must give their own examples. It is important that they go through the thought process – allow them to think of their own examples. Try to not help them, but you may guide them if they really need help; then let them think of more of their own examples.

You know that addition and subtraction are opposites and that they can also be used to check the answers of number sentences. Study the examples below, which show the opposite effect of addition and subtraction: • 58 – 58 = 0 • 264 – 264 = 0 • 37 – 4 + 4 = 37 • 27 + 6 – 6 = 27

Write down another 5 of your own examples.