(1 x 10^1) + (6 x 10^0) + (3/10^1) + (5/10^2)
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320
You don't have to test anything. Any number greater than 5 that ends in 5 is composite.
The next number in the series 3, 618, 72, is -1635. The relevant rule is Un = (-1161n2 + 4713n - 3546)/2 for n = 1, 2, 3, ...
To find all possible numbers using the digits 3, 5, 6, and 1, we can use the concept of permutations. There are 4 digits available, so there are 4 factorial (4!) ways to arrange them, which equals 24 possible combinations. This means there are 24 different numbers that can be formed using the digits 3, 5, 6, and 1.
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.