A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 115920 can be evenly divided by 1 2 3 4 5 6 7 8 9 10 12 14 15 16 18 20 21 23 24 28 30 35 36 40 42 45 46 48 56 60 63 69 70 72 80 84 90 92 105 112 115 120 126 138 140 144 161 168 180 184 207 210 230 240 252 276 280 315 322 336 345 360 368 414 420 460 483 504 552 560 630 644 690 720 805 828 840 920 966 1,008 1,035 1,104 1,260 1,288 1,380 1,449 1,610 1,656 1,680 1,840 1,932 2,070 2,415 2,520 2,576 2,760 2,898 3,220 3,312 3,864 4,140 4,830 5,040 5,520 5,796 6,440 7,245 7,728 8,280 9,660 11,592 12,880 14,490 16,560 19,320 23,184 28,980 38,640 57,960 and 115,920, with no remainder.
Since 115,920 cannot be divided by just 1 and 115,920, it is not a prime number.