A prime number is a natural number, greater than one, that can only be divided by 1 and itself.
The number 787500 can be evenly divided by 1 2 3 4 5 6 7 9 10 12 14 15 18 20 21 25 28 30 35 36 42 45 50 60 63 70 75 84 90 100 105 125 126 140 150 175 180 210 225 250 252 300 315 350 375 420 450 500 525 625 630 700 750 875 900 1,050 1,125 1,250 1,260 1,500 1,575 1,750 1,875 2,100 2,250 2,500 2,625 3,125 3,150 3,500 3,750 4,375 4,500 5,250 5,625 6,250 6,300 7,500 7,875 8,750 9,375 10,500 11,250 12,500 13,125 15,750 17,500 18,750 21,875 22,500 26,250 28,125 31,500 37,500 39,375 43,750 52,500 56,250 65,625 78,750 87,500 112,500 131,250 157,500 196,875 262,500 393,750 and 787,500, with no remainder.
Since 787,500 cannot be divided by just 1 and 787,500, it is not a prime number.