Answer:
9 and 6
Step-by-step explanation:
Write down the system.
The two numbers are x and y.
We are therefore given:
x+y=15
x-y=3
I will solve this for elimination since it is already setup for it.
I will add the columns since both equations are in the same form.
2x+0y=18
2x =18
Divide both sides by 2:
x =18/2
Simplify:
x =9
So if x+y=15 and x=9, then y=6.
The two numbers are 9 and 6.
Test:
9+6=15
9-6=3
Answer: 
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Explanation:
r = number of red pens
4r = cost of buying r red pens
example: if r = 10, then 4r = 4*10 = 40 is the total cost of buying 10 red pens
If John buys r red pens, then he must buy 20-r black pens so that r and 20-r add to 20.
John buys 20-r black pens, and at $5 each, so it will cost 5(20-r) dollars for just the black pens alone.
In total, John spends 4r + 5(20-r) dollars for all the pens (red & black)
We want $92 be the most he spends. This is the ceiling or highest value possible.
Therefore,

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Extra info:
If you want to solve for r, then

distribute

add r to both sides

subtract 92 from both sides


So John must buy at least 8 red pens. The most he can buy is 20 red pens since he wants 20 pens total.