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2415 = MMCDXV
40.25 hours or 40 hours 15 minutes.
2415
You can use any common multiple of 35 and 21, which are the multiples of the lowest common multiple of 35 and 21, namely: 105, 210, 315, 420, 525, 630, 735, 840, 945, 1050, 1155, 1260, 1365, 1470, 1575, 1680, 1785, 1890, 1995, 2100, 2205, 2310, 2415, 2520, 2625, 2730, 2835, 2940, 3045, 3150, 3255, 3360, 3465, 3570, 3675, 3780, 3885, 3990, 4095, 4200, 4305, 4410, 4515, 4620, 4725, 4830, 4935, 5040, 5145, 5250, 5355, 5460, 5565, 5670, 5775, 5880, 5985, 6090, 6195, 6300, 6405, 6510, 6615, 6720, 6825, 6930, 7035, 7140, 7245, 7350, 7455, 7560, 7665, 7770, 7875, 7980, 8085, 8190, 8295, 8400, 8505, 8610, 8715, 8820, 8925, 9030, 9135, 9240, 9345, 9450, 9555, 9660, 9765, 9870, 9975, 10080, 10185, 10290, 10395, 10500, ...
There are a great number of them (450 in fact). All multiples of the lowest common multiple of 3 and 5 (which is 15) which are not multiples of the lcm of 3, 4 and 5 (which is 60) will solve the problem, thus any of the four digit numbers, namely: 1005, 1035, 1050, 1065, 1095, 1110, 1125, 1155, 1170, 1185, 1215, 1230, 1245, 1275, 1290, 1305, 1335, 1350, 1365, 1395, 1410, 1425, 1455, 1470, 1485, 1515, 1530, 1545, 1575, 1590, 1605, 1635, 1650, 1665, 1695, 1710, 1725, 1755, 1770, 1785, 1815, 1830, 1845, 1875, 1890, 1905, 1935, 1950, 1965, 1995, 2010, 2025, 2055, 2070, 2085, 2115, 2130, 2145, 2175, 2190, 2205, 2235, 2250, 2265, 2295, 2310, 2325, 2355, 2370, 2385, 2415, 2430, 2445, 2475, 2490, 2505, 2535, 2550, 2565, 2595, 2610, 2625, 2655, 2670, 2685, 2715, 2730, 2745, 2775, 2790, 2805, 2835, 2850, 2865, 2895, 2910, 2925, 2955, 2970, 2985, 3015, 3030, 3045, 3075, 3090, 3105, 3135, 3150, 3165, 3195, 3210, 3225, 3255, 3270, 3285, 3315, 3330, 3345, 3375, 3390, 3405, 3435, 3450, 3465, 3495, 3510, 3525, 3555, 3570, 3585, 3615, 3630, 3645, 3675, 3690, 3705, 3735, 3750, 3765, 3795, 3810, 3825, 3855, 3870, 3885, 3915, 3930, 3945, 3975, 3990, 4005, 4035, 4050, 4065, 4095, 4110, 4125, 4155, 4170, 4185, 4215, 4230, 4245, 4275, 4290, 4305, 4335, 4350, 4365, 4395, 4410, 4425, 4455, 4470, 4485, 4515, 4530, 4545, 4575, 4590, 4605, 4635, 4650, 4665, 4695, 4710, 4725, 4755, 4770, 4785, 4815, 4830, 4845, 4875, 4890, 4905, 4935, 4950, 4965, 4995, 5010, 5025, 5055, 5070, 5085, 5115, 5130, 5145, 5175, 5190, 5205, 5235, 5250, 5265, 5295, 5310, 5325, 5355, 5370, 5385, 5415, 5430, 5445, 5475, 5490, 5505, 5535, 5550, 5565, 5595, 5610, 5625, 5655, 5670, 5685, 5715, 5730, 5745, 5775, 5790, 5805, 5835, 5850, 5865, 5895, 5910, 5925, 5955, 5970, 5985, 6015, 6030, 6045, 6075, 6090, 6105, 6135, 6150, 6165, 6195, 6210, 6225, 6255, 6270, 6285, 6315, 6330, 6345, 6375, 6390, 6405, 6435, 6450, 6465, 6495, 6510, 6525, 6555, 6570, 6585, 6615, 6630, 6645, 6675, 6690, 6705, 6735, 6750, 6765, 6795, 6810, 6825, 6855, 6870, 6885, 6915, 6930, 6945, 6975, 6990, 7005, 7035, 7050, 7065, 7095, 7110, 7125, 7155, 7170, 7185, 7215, 7230, 7245, 7275, 7290, 7305, 7335, 7350, 7365, 7395, 7410, 7425, 7455, 7470, 7485, 7515, 7530, 7545, 7575, 7590, 7605, 7635, 7650, 7665, 7695, 7710, 7725, 7755, 7770, 7785, 7815, 7830, 7845, 7875, 7890, 7905, 7935, 7950, 7965, 7995, 8010, 8025, 8055, 8070, 8085, 8115, 8130, 8145, 8175, 8190, 8205, 8235, 8250, 8265, 8295, 8310, 8325, 8355, 8370, 8385, 8415, 8430, 8445, 8475, 8490, 8505, 8535, 8550, 8565, 8595, 8610, 8625, 8655, 8670, 8685, 8715, 8730, 8745, 8775, 8790, 8805, 8835, 8850, 8865, 8895, 8910, 8925, 8955, 8970, 8985, 9015, 9030, 9045, 9075, 9090, 9105, 9135, 9150, 9165, 9195, 9210, 9225, 9255, 9270, 9285, 9315, 9330, 9345, 9375, 9390, 9405, 9435, 9450, 9465, 9495, 9510, 9525, 9555, 9570, 9585, 9615, 9630, 9645, 9675, 9690, 9705, 9735, 9750, 9765, 9795, 9810, 9825, 9855, 9870, 9885, 9915, 9930, 9945, 9975, 9990 take your pick.