2.1乘以8又二分之一乘以5.5乘以六又二分之一除以（六又五分之三乘以9.1乘以0.51）
=2.1×8又1/2×5.5×6又1/2÷(6又3/5×9.1×0.51)
=21/10×17/2×11/2×13/2÷(33/5×91/10×51/100)
=(21×17×11×13)/80×5000/(33×91×51)---(21×17与51,11与33,13与91,5000与80分别约分)
=7×125/(2×7)
=125/2
=62又1/2.太感谢了！！！能不能再帮我解一道题。！（3.5乘以69减3.4）分之（1999乘以（3.4乘以69加3.5））谢谢啦！！！［1999×(3.4×69+3.5)］÷［3.5×69-3.4］=［1999×(34×69+35)/10］÷［(35×69-34)/10］=［1999×(34×69+34+1)］÷(34×69+69-34)=1999×(34×70+1)÷(34×70)=1999+1999/2380=1999又1999/2380。