A train is 216 m long. It crosses a platform in 19 seconds with speed 21 m/s. If some 21 m long boxes are added in the train then it crosses the same platform in 27 seconds at the same speed. Then find the number of boxes that were added in the train.

Distance to cross platform = Train length + Platform length. At 21 m/s for 19 seconds, total distance = 21 × 19 = 399 m. Platform length = 399 - 216 = 183 m. After adding n boxes of 21 m each, new train length = 216 + 21n. Total distance = (216 + 21n) + 183 = 399 + 21n. At same speed for 27 seconds: 21 × 27 = 567 = 399 + 21n. Therefore 21n = 168 and n = 8 boxes.