+0  
 
0
104
2
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How many 3-digit numbers have the property that the units digit is at least twice the tens digit?

 Aug 13, 2020
 #1
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+1

(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

 Aug 13, 2020
 #2
avatar+111628 
+1

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

 Aug 13, 2020

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