How did you get " all ≤ 28000" when the total = 30000?

Because the additional requirement that each one needs to have at least 1000 would mean that the most a builder can buy is 28000 bricks.

So you meant each of P,Q,R ≤ 28000

But the equations have already fixed Q = 10000.

So it should be 1000 P ≤ 19000 or 1000 ≤ R ≤ 19000

Now that you mention it...

I forgot that I have already set Q to 10000.