1/3-（0.875乘2/13加1除以6.5除以8）乘1又6/7等于多少?有算式

1/3-（0.875乘2/13加1除以6.5除以8）乘1又6/7
=1/3-(7/8x2/13+1÷13/2÷8)x13/7
=1/3-(7/52+1/52)x13/7
=1/3-2/13x13/7
=1/3-2/7
=1/21
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