Round to one or two leading digits, compute, and match the order of magnitude. The correct answer is ≈ 700. Traps to avoid: ≈ 800 comes from the "rounded up too far" error; ≈ 1,000 comes from the "wrong place value" error; ≈ 500 comes from the "under estimate" error; ≈ 600 comes from the "rounded down too far" error.