Answer
$5\ days$
Work Step by Step
Step 1. Assume they should wait $x$ days, the total weight would be $12,000+400x=400(30+x)$
Step 2. The price per 100 pound would be $4-0.1x=0.1(40-x)$
Step 3. The revenue per tree would be $R=400(30+x)(0.1)(40-x)/100=0.4(30+x)(40-x)=0.4(1200+10x-x^2)$
Step 4. Let $R=490$, we have $0.4(1200+10x-x^2)=490$ or $x^2-10x+25=0$ which gives $x=5\ days$