How many 3-digit numbers have the property that the units digit is at least twice the tens digit?
(102, 103, 104, 105, 106, 107, 108, 109, 112, 113, 114, 115, 116, 117, 118, 119, 124, 125, 126, 127, 128, 129, 136, 137, 138, 139, 148, 149, 202, 203, 204, 205, 206, 207, 208, 209, 212, 213, 214, 215, 216, 217, 218, 219, 224, 225, 226, 227, 228, 229, 236, 237, 238, 239, 248, 249, 302, 303, 304, 305, 306, 307, 308, 309, 312, 313, 314, 315, 316, 317, 318, 319, 324, 325, 326, 327, 328, 329, 336, 337, 338, 339, 348, 349, 402, 403, 404, 405, 406, 407, 408, 409, 412, 413, 414, 415, 416, 417, 418, 419, 424, 425, 426, 427, 428, 429, 436, 437, 438, 439, 448, 449, 502, 503, 504, 505, 506, 507, 508, 509, 512, 513, 514, 515, 516, 517, 518, 519, 524, 525, 526, 527, 528, 529, 536, 537, 538, 539, 548, 549, 602, 603, 604, 605, 606, 607, 608, 609, 612, 613, 614, 615, 616, 617, 618, 619, 624, 625, 626, 627, 628, 629, 636, 637, 638, 639, 648, 649, 702, 703, 704, 705, 706, 707, 708, 709, 712, 713, 714, 715, 716, 717, 718, 719, 724, 725, 726, 727, 728, 729, 736, 737, 738, 739, 748, 749, 802, 803, 804, 805, 806, 807, 808, 809, 812, 813, 814, 815, 816, 817, 818, 819, 824, 825, 826, 827, 828, 829, 836, 837, 838, 839, 848, 849, 902, 903, 904, 905, 906, 907, 908, 909, 912, 913, 914, 915, 916, 917, 918, 919, 924, 925, 926, 927, 928, 929, 936, 937, 938, 939, 948, 949) = Total 252
+118667 What about 100? I think you have missed many others as well.
if the middle digit is 0 the last digit can be any 10
if the middle digit is 1 the last digit can be 2 or bigger 8
if the middle digit is 2 the last digit can be 4 or bigger 6
if the middle digit is 3 the last digit can be 6 or bigger 4
if the middle digit is 5 the last digit can be 8 or bigger 2
Total so far = 30
the first digit can be 1 to 9
30*9=270
