80×79×78×77×76×75×74×73×72×71×70×69×68×67×66×65×64×63×62×61➗
(1×2×3×4×5×6×7×8×9×10×11×12×13×14×15×16×17×18×19×20)=
[(4)(79/19)(13/3)(77/17)(19/4)(5)]×
[(37/7)(73/13)(6)(71/11)(7)(23/3)(17/2)]×
[(67/7)(11)(13)(16)(21)(31)(61)]=
395×30875131×289882736=
395(30870000+5131)(289800000+82736)=
(400-5)×
(8,946,126,000,000,000+
2,554,060,320,000+
1,486,963,800,000+
424,518,416)=(400-5)×
(8,949,000,000,000,000+
1,166,000,000,000+
1,447,000,000+
1,638,416)=
(400-5)(8,950,167,448,638,416)=
3,580,066,979,455,366,400-
44,750,837,243,192,080=
3,535,316,142,212,174,320.
This is the number of combinations.