8/15×2又1/2÷(6又2/3×0.28+6又2/3×72%)
=8/15×5/2÷[6又2/3×（0.28+0.72）]
=4/3÷6又2/3
=4/3×3/20
=1/51/4×0.84-0.26×25%+0.42÷4等于多少？1/4×0.84-0.26×25%+0.42÷4=0.25×0.84-0.26×0.25+0.42×0.25=0.25×（0.84-0.26+0.42）=0.25×1=0.251又1/2+2又1/6+3又1/12+……+9又1/90等于多少？1又1/2+2又1/6+3又1/12+……+9又1/90=（1+2+3+。。。。9）+（1/2+1/6+...1/90)=45+(1-1/2+1/2-1/3+1/3-1/4+....1/9-1/10)=45+1-1/10=45+9/10=45又9/10