599. 530 м 900 мм + 420 мм

To solve this problem, we need to add the lengths given. First, we add the millimeters: 900 mm + 420 mm = 1320 mm. Since 1320 mm is equal to 1 meter and 320 millimeters, we carry over 1 meter to the meter sum. Then, we add the meters: 530 m + 1 m (carried over) = 531 m. The remaining millimeters are 320 mm. The total length is 531 m and 320 mm. Alternatively, if we convert everything to millimeters first: 530 m = 530,000 mm. So, 530,000 mm + 900 mm + 420 mm = 531,320 mm. Converting back to meters and millimeters: 531,320 mm = 531 m and 320 mm.