A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 1640880 can be evenly divided by 1 2 3 4 5 6 8 9 10 12 15 16 18 20 24 30 36 40 43 45 48 53 60 72 80 86 90 106 120 129 144 159 172 180 212 215 240 258 265 318 344 360 387 424 430 477 516 530 636 645 688 720 774 795 848 860 954 1,032 1,060 1,272 1,290 1,548 1,590 1,720 1,908 1,935 2,064 2,120 2,279 2,385 2,544 2,580 3,096 3,180 3,440 3,816 3,870 4,240 4,558 4,770 5,160 6,192 6,360 6,837 7,632 7,740 9,116 9,540 10,320 11,395 12,720 13,674 15,480 18,232 19,080 20,511 22,790 27,348 30,960 34,185 36,464 38,160 41,022 45,580 54,696 68,370 82,044 91,160 102,555 109,392 136,740 164,088 182,320 205,110 273,480 328,176 410,220 546,960 820,440 and 1,640,880, with no remainder.
Since 1,640,880 cannot be divided by just 1 and 1,640,880, it is not a prime number.