A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 405720 can be evenly divided by 1 2 3 4 5 6 7 8 9 10 12 14 15 18 20 21 23 24 28 30 35 36 40 42 45 46 49 56 60 63 69 70 72 84 90 92 98 105 115 120 126 138 140 147 161 168 180 184 196 207 210 230 245 252 276 280 294 315 322 345 360 392 414 420 441 460 483 490 504 552 588 630 644 690 735 805 828 840 882 920 966 980 1,035 1,127 1,176 1,260 1,288 1,380 1,449 1,470 1,610 1,656 1,764 1,932 1,960 2,070 2,205 2,254 2,415 2,520 2,760 2,898 2,940 3,220 3,381 3,528 3,864 4,140 4,410 4,508 4,830 5,635 5,796 5,880 6,440 6,762 7,245 8,280 8,820 9,016 9,660 10,143 11,270 11,592 13,524 14,490 16,905 17,640 19,320 20,286 22,540 27,048 28,980 33,810 40,572 45,080 50,715 57,960 67,620 81,144 101,430 135,240 202,860 and 405,720, with no remainder.
Since 405,720 cannot be divided by just 1 and 405,720, it is not a prime number.