①:(1/2-1/3)X3/4除1/4
=(1/2-1/3)×3/4×4
=(1/2-1/3)×3
=3/2-1= 1/2
②:35×25%+1/4×27+38除4
=35×1/4+1/4×27+38×1/4
=1/4×(35+27+38)
=25
③:28.8-640%-2.8-4.6
=28.8-2.8-64-4.6
=26-64-4.6
=-42.6
④:(15-5)X5/13+8/15X10
=10×5/13+8/15×10
=10×(5/13+8/15) 这个地方是不是错了?8/15可能是8/13?
=358/39
⑤:x:0.42=2/3
x=0.28