Answer: 
pairs
Step-by-step explanation:
Given
Profit on selling 'x' pairs of shoes is 
Find the derivative of the function to find out the critical points
Thus, for 4 pair of shoes profit is maximize.
Your brother would be nine years old.
~You would set up the equation x+(3x+5)=6565
~Combine like products so 3x+x=4x.
~Now you have 4x+5=6565.
~Isolate the variable by subracting the 5 from both sides, so you have 4x=6560.
~Since you multiply x by 4, divide both sides by 4, and 6560 divided by 4 is 1,640.
*The first number is 1,640.
~Now to find the second number you simply plug in 1,640 to (3x+5), with 1,640 being x.
~That would become [3(1,640)+5].
~3 times 1, 640 is 4,920, and add the 5 so you get 4,925.
*The second and larger number is 4,925.
If you wanna check just add the two numbers, and you get 6565.
