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Is '5 less than a number' the same as '5 less a number'? 

No, '5 less than a number N" means to start with the
number N and make another number out of it which is 5 
less.  To do that you start with N and subtract 5. So you 
put down N and then make it less by subtracting 5
from it:

              N - 5

'5 less a number' means to start with the 5 and 
subtract a number N from it.  So you put down 5 and
then subtract a number N from it.

              5 - N

Edwin