有一项工程,甲单独做要10天完成,乙单独做要12天完成,丙单独做要15天完成.甲乙丙合作了3天后
1个回答

[1-3(1/10+1/12+1/15)]÷(1/12+1/15)

=[1-3(6/60+5/60+4/60)]÷(5/60+4/60)

=[1-3(15/60)]÷(9/60)

=[1-3(1/4)]÷(3/20)

=[1-3/4](20/3)

=(1/4)(20/3)

=5/3

需要5/3天才能完成.