（78.6-0.786×25+75％×21.4）÷2001
=（78.6-78.6×0.25+75％×21.4）÷2001
=（78.6×0.75+0.75×21.4）÷2001
=（(78.6+21.4）×0.75）÷2001
=（100×0.75）÷2001
=75/2001
=25/667