a) Có thể lập được 3 chữ số từ các số 0, 1, 2, 3, 4, 5, 6 là: 012, 013, 014, 015, 016, 023, 024, 025, 026, 034, 035, 036, 045, 046, 056, 123, 124, 125, 126, 134, 135, 136, 145, 146, 156, 234, 235, 236, 245, 246, 256, 345, 346, 356, 456.
b) Có thể lập được 4 chữ số khác nhau từ các số 0, 1, 2, 3, 4, 5, 6 là: 0123, 0124, 0125, 0126, 0132, 0134, 0135, 0136, 0142, 0143, 0145, 0146, 0152, 0153, 0154, 0156, 0162, 0163, 0164, 0165, 0231, 0234, 0235, 0236, 0241, 0243, 0245, 0246, 0251, 0253, 0254, 0256, 0261, 0263, 0264, 0265, 0312, 0314, 0315, 0316, 0321, 0324, 0325, 0326, 0341, 0342, 0345, 0346, 0351, 0352, 0354, 0356, 0361, 0362, 0364, 0365, 0412, 0413, 0415, 0416, 0421, 0423, 0425, 0426, 0431, 0432, 0435, 0436, 0451, 0452, 0453, 0456, 0461, 0462, 0463, 0465, 0512, 0513, 0514, 0516, 0521, 0523, 0524, 0526, 0531, 0532, 0534, 0536, 0541, 0542, 0543, 0546, 0561, 0562, 0563, 0564, 0612, 0613, 0614, 0615, 0621, 0623, 0624, 0625, 0631, 0632, 0634, 0635, 0641, 0642, 0643, 0645, 0651, 0652, 0653, 0654, 1230, 1234, 1235, 1236, 1240, 1243, 1245, 1246, 1250, 1253, 1254, 1256, 1260, 1263, 1264, 1265, 1302, 1304, 1305, 1306, 1320, 1324, 1325, 1326, 1340, 1342, 1345, 1346, 1350, 1352, 1354, 1356, 1360, 1362, 1364, 1365, 1402, 1403, 1405, 1406, 1420, 1423, 1425, 1426, 1430, 1432, 1435, 1436, 1450, 1452, 1453, 1456, 1460, 1462, 1463, 1465, 1502, 1503, 1504, 1506, 1520, 1523, 1524, 1526, 1530, 1532, 1534, 1536, 1540, 1542, 1543, 1546, 1560, 1562, 1563, 1564, 1602, 1603, 1604, 1605, 1620, 1623, 1624, 1625, 1630, 1632, 1634,