1) 8 * 10 000 + 7 * 1000 + 2 * 100 + 1 * 10 = 80 000 + 7 000 + 200 + 10 = 87210
2) 9 * 100 000 + 0 * 10 000 + 0 * 1000 + 1 * 100 + 0 *10 + 5 = 900 000 + 0 + 0 + 100 + 0 + 5 = 900 105
3) а * 100 + 7 *10 + n = 100a + 70 + n
4) x * 1000 + y * 100 + z * 10 + a = 1000x + 100y + 10z + a