One number is 4 less than twice a second number. find a pair of such numbers so that their product is as small as possible.

 For this case, the first thing we must do is define a variable.
 We have then:
 x: unknown number.
 First number: 2x - 4
 Second number: x
 The product of the numbers is:
 
 Rewriting:
 
 Deriving the equation we have:
 
 We equal zero and clear x:
 
 Then, the first number is:
 
 
 First number: -2
 Second number: 1