The number 645,120 can be evenly divided by 1 2 3 4 5 6 7 8 9 10 12 14 15 16 18 20 21 24 28 30 32 35 36 40 42 45 48 56 60 63 64 70 72 80 84 90 96 105 112 120 126 128 140 144 160 168 180 192 210 224 240 252 256 280 288 315 320 336 360 384 420 448 480 504 512 560 576 630 640 672 720 768 840 896 960 1,008 1,024 1,120 1,152 1,260 1,280 1,344 1,440 1,536 1,680 1,792 1,920 2,016 2,048 2,240 2,304 2,520 2,560 2,688 2,880 3,072 3,360 3,584 3,840 4,032 4,480 4,608 5,040 5,120 5,376 5,760 6,144 6,720 7,168 7,680 8,064 8,960 9,216 10,080 10,240 10,752 11,520 13,440 14,336 15,360 16,128 17,920 18,432 20,160 21,504 23,040 26,880 30,720 32,256 35,840 40,320 43,008 46,080 53,760 64,512 71,680 80,640 92,160 107,520 129,024 161,280 215,040 322,560 and 645,120, with no remainder.
Since 645,120 cannot be divided by just 1 and 645,120, it is a composite number.