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When we substitute a number for a letter, we simply replace the letter with the number.

 

Example

Work out the value of the following expression when a = 4 and b = 9

 

3a + 4b
2 3

 

Answer

We substitute a = 4 and b = 9 into the following expression:

 

3a + 4b
2 3

  

= 3 x 4 + 4 x 9
2  3

 

= 12 + 36
2  3

 

=  6  +  12  =  18

 

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