LCM for 21 and 42

The first method to find LCM for numbers 21 and 42 is to find Greatest Common Factor (GCF) of these numbers. Here's the formula:

LCM = (Number1 × Number2) ÷ GCF

GCF of numbers 21 and 42 is 21, so

LCM = (21 × 42) ÷ 21

LCM = 882 ÷ 21

LCM = 42

The second method to find LCM for numbers 21 and 42 is to list out the common multiples for both nubmers and pick the first which matching:

Multiples of 21: 21, 42, 63, 84

Multiples of 42: 42, 84, 126, 168, 210, 252, 294, 336, 378, 420, 462, 504, 546, 588, 630, 672, 714, 756, 798, 840, 882, 924, 966, 1008, 1050, 1092, 1134, 1176, 1218, 1260, 1302, 1344, 1386, 1428, 1470, 1512, 1554, 1596, 1638, 1680, 1722, 1764, 1806, 1848, 1890, 1932, 1974, 2016, 2058, 2100, 2142, 2184, 2226, 2268, 2310, 2352, 2394, 2436, 2478, 2520, 2562, 2604, 2646, 2688, 2730, 2772, 2814, 2856, 2898, 2940, 2982, 3024, 3066, 3108, 3150, 3192, 3234, 3276, 3318, 3360, 3402, 3444, 3486, 3528, 3570, 3612, 3654, 3696, 3738, 3780, 3822, 3864, 3906, 3948, 3990, 4032, 4074, 4116, 4158, 4200, 4242, 4284, 4326, 4368, 4410, 4452, 4494, 4536, 4578, 4620, 4662, 4704, 4746, 4788, 4830, 4872, 4914, 4956, 4998, 5040, 5082, 5124, 5166, 5208, 5250, 5292, 5334, 5376, 5418, 5460, 5502, 5544, 5586, 5628, 5670, 5712, 5754, 5796, 5838, 5880, 5922, 5964, 6006, 6048, 6090, 6132, 6174, 6216, 6258, 6300, 6342, 6384, 6426, 6468, 6510, 6552, 6594, 6636, 6678, 6720, 6762, 6804, 6846, 6888, 6930, 6972, 7014, 7056, 7098, 7140, 7182, 7224, 7266, 7308, 7350, 7392, 7434, 7476, 7518, 7560, 7602, 7644, 7686, 7728, 7770, 7812, 7854, 7896, 7938, 7980, 8022, 8064, 8106, 8148, 8190, 8232, 8274, 8316, 8358, 8400, 8442, 8484, 8526, 8568, 8610, 8652, 8694, 8736, 8778, 8820, 8862, 8904, 8946, 8988, 9030, 9072, 9114, 9156, 9198, 9240, 9282, 9324, 9366, 9408, 9450, 9492, 9534, 9576, 9618, 9660, 9702, 9744, 9786, 9828, 9870, 9912, 9954, 9996, 10038, 10080, 10122, 10164, 10206, 10248, 10290, 10332, 10374, 10416, 10458, 10500, 10542, 10584, 10626, 10668, 10710, 10752, 10794, 10836, 10878, 10920, 10962, 11004, 11046, 11088, 11130, 11172, 11214, 11256, 11298, 11340, 11382, 11424, 11466, 11508, 11550, 11592, 11634, 11676, 11718, 11760, 11802, 11844, 11886, 11928, 11970, 12012, 12054, 12096, 12138, 12180, 12222, 12264, 12306, 12348, 12390, 12432, 12474, 12516, 12558, 12600, 12642, 12684, 12726, 12768, 12810, 12852, 12894, 12936, 12978, 13020, 13062, 13104, 13146, 13188, 13230, 13272, 13314, 13356, 13398, 13440, 13482, 13524, 13566, 13608, 13650, 13692, 13734, 13776, 13818, 13860, 13902, 13944, 13986, 14028, 14070, 14112, 14154, 14196, 14238, 14280, 14322, 14364, 14406, 14448, 14490, 14532, 14574, 14616, 14658, 14700, 14742, 14784, 14826, 14868, 14910, 14952, 14994, 15036, 15078, 15120, 15162, 15204, 15246, 15288, 15330, 15372, 15414, 15456, 15498, 15540, 15582, 15624, 15666, 15708, 15750, 15792, 15834, 15876, 15918, 15960, 16002, 16044, 16086, 16128, 16170, 16212, 16254, 16296, 16338, 16380, 16422, 16464, 16506, 16548, 16590, 16632, 16674, 16716, 16758, 16800, 16842, 16884, 16926, 16968, 17010, 17052, 17094, 17136, 17178, 17220, 17262, 17304, 17346, 17388, 17430, 17472, 17514, 17556, 17598, 17640, 17682, 17724, 17766, 17808, 17850, 17892, 17934, 17976, 18018, 18060, 18102, 18144, 18186, 18228, 18270, 18312, 18354, 18396, 18438, 18480, 18522, 18564, 18606, 18648, 18690, 18732, 18774, 18816, 18858, 18900, 18942, 18984, 19026, 19068, 19110, 19152, 19194, 19236, 19278, 19320, 19362, 19404, 19446, 19488, 19530, 19572, 19614, 19656, 19698, 19740, 19782, 19824, 19866, 19908, 19950, 19992, 20034, 20076, 20118, 20160, 20202, 20244, 20286, 20328, 20370, 20412, 20454, 20496, 20538, 20580, 20622, 20664, 20706, 20748, 20790, 20832, 20874, 20916, 20958, 21000, 21042, 21084, 21126, 21168, 21210, 21252, 21294, 21336, 21378, 21420, 21462, 21504, 21546, 21588, 21630, 21672, 21714, 21756, 21798, 21840, 21882, 21924, 21966, 22008, 22050, 22092, 22134, 22176, 22218, 22260, 22302, 22344, 22386, 22428, 22470, 22512, 22554, 22596, 22638, 22680, 22722, 22764, 22806, 22848, 22890, 22932, 22974, 23016, 23058, 23100, 23142, 23184, 23226, 23268, 23310, 23352, 23394, 23436, 23478, 23520, 23562, 23604, 23646, 23688, 23730, 23772, 23814, 23856, 23898, 23940, 23982, 24024, 24066, 24108, 24150, 24192, 24234, 24276, 24318, 24360, 24402, 24444, 24486, 24528, 24570, 24612, 24654, 24696, 24738, 24780, 24822, 24864, 24906, 24948, 24990, 25032, 25074, 25116, 25158, 25200, 25242, 25284, 25326, 25368, 25410, 25452, 25494, 25536, 25578, 25620, 25662, 25704, 25746, 25788, 25830, 25872, 25914, 25956, 25998, 26040, 26082, 26124, 26166, 26208, 26250, 26292, 26334, 26376, 26418, 26460, 26502, 26544, 26586, 26628, 26670, 26712, 26754, 26796, 26838, 26880, 26922, 26964, 27006, 27048, 27090, 27132, 27174, 27216, 27258, 27300, 27342, 27384, 27426, 27468, 27510, 27552, 27594, 27636, 27678, 27720, 27762, 27804, 27846, 27888, 27930, 27972, 28014, 28056, 28098, 28140, 28182, 28224, 28266, 28308, 28350, 28392, 28434, 28476, 28518, 28560, 28602, 28644, 28686, 28728, 28770, 28812, 28854, 28896, 28938, 28980, 29022, 29064, 29106, 29148, 29190, 29232, 29274, 29316, 29358, 29400, 29442, 29484, 29526, 29568, 29610, 29652, 29694, 29736, 29778, 29820, 29862, 29904, 29946, 29988, 30030, 30072, 30114, 30156, 30198, 30240, 30282, 30324, 30366, 30408, 30450, 30492, 30534, 30576, 30618, 30660, 30702, 30744, 30786, 30828, 30870, 30912, 30954, 30996, 31038, 31080, 31122, 31164, 31206, 31248, 31290, 31332, 31374, 31416, 31458, 31500, 31542, 31584, 31626, 31668, 31710, 31752, 31794, 31836, 31878, 31920, 31962, 32004, 32046, 32088, 32130, 32172, 32214, 32256, 32298, 32340, 32382, 32424, 32466, 32508, 32550, 32592, 32634, 32676, 32718, 32760, 32802, 32844, 32886, 32928, 32970, 33012, 33054, 33096, 33138, 33180, 33222, 33264, 33306, 33348, 33390, 33432, 33474, 33516, 33558, 33600, 33642, 33684, 33726, 33768, 33810, 33852, 33894, 33936, 33978, 34020, 34062, 34104, 34146, 34188, 34230, 34272, 34314, 34356, 34398, 34440, 34482, 34524, 34566, 34608, 34650, 34692, 34734, 34776, 34818, 34860, 34902, 34944, 34986, 35028, 35070, 35112, 35154, 35196, 35238, 35280, 35322, 35364, 35406, 35448, 35490, 35532, 35574, 35616, 35658, 35700, 35742, 35784, 35826, 35868, 35910, 35952, 35994, 36036, 36078, 36120, 36162, 36204, 36246, 36288, 36330, 36372, 36414, 36456, 36498, 36540, 36582, 36624, 36666, 36708, 36750, 36792, 36834, 36876, 36918, 36960, 37002, 37044, 37086, 37128, 37170, 37212, 37254, 37296, 37338, 37380, 37422, 37464, 37506, 37548, 37590, 37632, 37674, 37716, 37758, 37800, 37842, 37884, 37926, 37968, 38010, 38052, 38094, 38136, 38178, 38220, 38262, 38304, 38346, 38388, 38430, 38472, 38514, 38556, 38598, 38640, 38682, 38724, 38766, 38808, 38850, 38892, 38934, 38976, 39018, 39060, 39102, 39144, 39186, 39228, 39270, 39312, 39354, 39396, 39438, 39480, 39522, 39564, 39606, 39648, 39690, 39732, 39774, 39816, 39858, 39900, 39942, 39984, 40026, 40068, 40110, 40152, 40194, 40236, 40278, 40320, 40362, 40404, 40446, 40488, 40530, 40572, 40614, 40656, 40698, 40740, 40782, 40824, 40866, 40908, 40950, 40992, 41034, 41076, 41118, 41160, 41202, 41244, 41286, 41328, 41370, 41412, 41454, 41496, 41538, 41580, 41622, 41664, 41706, 41748, 41790, 41832, 41874, 41916, 41958, 42000, [...], 42

So the LCM for 21 and 42 is 42

Another method to find LCM for numbers 21 and 42 is to list all Prime Factors for both numbers and multiply the highest exponent prime factors: