Evaluating expressions, step by step
Before a letter can be solved for, it has to be substituted.
The question
Work out the value of the expression.
2x − 8, x = 5
2x − 8withx = 5
2 × 5=10
10−8=2
2x − 8=2
- A letter is a number that has not been written down yet, and this question writes it down: x = 5. So put the value in where the letter is — in BRACKETS, which keeps whatever was being done to the letter being done to the number.
- 2x means 2 TIMES x, and multiplying happens before adding or taking away however the line is written. So this multiplication comes first: 2 × 5 = 10.
- Now the last bit: 10 − 8 = 2. Doing it the other way round — taking away first and multiplying after — gives -6, which is why the order is the whole of this question.
- So 2x − 8 is 2 when x = 5.
How it works.
- 01
A letter is a number that has not been written down yet, and this question writes it down: x = 5. So put the value in where the letter is — in BRACKETS, which keeps whatever was being done to the letter being done to the number.
- 02
2x means 2 TIMES x, and multiplying happens before adding or taking away however the line is written. So this multiplication comes first: 2 × 5 = 10.
- 03
Now the last bit: 10 − 8 = 2. Doing it the other way round — taking away first and multiplying after — gives -6, which is why the order is the whole of this question.
- 04
So 2x − 8 is 2 when x = 5.