Are there platform boards showing 15, and 10A? In that case Sealdah has platform nos. 1, 1A, 2, 3, 4, 4A, 5, 6, 7, 8, 9A, 9B, 9C, 10, 10A, 11, 12, 13, 14, 15 and they add up to 20. And if it has platform no. 9, the total number of platforms at SDAH move even further adding 21.