When we substitute a number for a letter, we simply replace the letter with the number.
However, we must remember that 4b means 4 x b.
Example
Work out the value of the following expression when a = 4
3a - 7
Answer
3a just means 3 x a , so we have
3 x a - 7
Change the letter (a) into the number we've been given (4)
3 x 4 - 7
Now multiply first - remember the order of BIDMAS (or BODMAS)
12 - 7 = 5
Did that make sense?
The key to substitution in algebra is to work slowly, one step at a time.
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