P(600-30 < X < 600+30) = P(570<X<630) = \Phi (\frac{630-600}{\sqrt{360}}) - \Phi (\frac{570-600}{\sqrt{360}}) \approx 2\cdot \Phi(1.58) - 1 \approx 2\cdot 0.9429 - 1 = 0.8858 = 88,58\%.