For word problems like this one, always focus on the actual question being asked: "How many orange seeds?" and see if we can rewrite things in terms of that value. That way we can work with one variable instead of two.

So, rewrite apple seeds as (4 * the number of orange seeds) or a = 4o.

Now, the total number of trees, 420, is just a percentage of the apple seeds + a percentage of the orange seeds:

15% of the apple seeds + 10% of the orange seeds = 420.

Doing the math, we have:

\( .15(4o) + .1(o) = 420\)
.\(6o + .1o = 420\)
\(.7o = 420 \)
\(o = 600\)

Answer: B