ʇɐɥʇ ɥuɐɹ uɐq ɔɐɔ ɐl ƃunp ıɥʇ ʎɐp uǝp ɔonp ɔop uɐq ɔɐɔ ɐl ʇǝıq ɥuıɯ ƃunɥu 'ɔonp ɔop ıoɯ ıɐl ɔonƃu ʎɐox ıɐɥd ɐʌ ɔop oɥʞ ɐl ʇɐɹ ıɥʇ ʎɐu ǝɥʇ ʇǝıʌ ɐl ʇǝıq ɥuıɯ
TBR, ta có:
11 x 12 x 13 x.....x 28 x 29.
Ta thấy :
15 x S chẵn =.....0 ( 1 cs 0)
20 x 21 = 420 ( 1 cs 0)
25 x 24 = 600(2 cs 0)
=> Tích trên có: 1 + 2 + 1 = 4 ( cs 0)