B1+3B2+4B3+5B4+6B5=1876 (1)B1+5B2+7B3+9B4+11B5=2984(2) (2)-(1)得2B2+3B3+4B4+5B5=1108 (3)(1)-(3)得B1+B2+B3+B4+B5=768
1876*2-2984=768
56879