A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 156240 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 31 35 36 40 42 45 48 56 60 62 63 70 72 80 84 90 93 105 112 120 124 126 140 144 155 168 180 186 210 217 240 248 252 279 280 310 315 336 360 372 420 434 465 496 504 558 560 620 630 651 720 744 840 868 930 1,008 1,085 1,116 1,240 1,260 1,302 1,395 1,488 1,680 1,736 1,860 1,953 2,170 2,232 2,480 2,520 2,604 2,790 3,255 3,472 3,720 3,906 4,340 4,464 5,040 5,208 5,580 6,510 7,440 7,812 8,680 9,765 10,416 11,160 13,020 15,624 17,360 19,530 22,320 26,040 31,248 39,060 52,080 78,120 and 156,240, with no remainder.
Since 156,240 cannot be divided by just 1 and 156,240, it is not a prime number.