`a) 1+2³+3³–4²× n=20`
`=>1+8+27-16n=20`
`=>36-16n=20`
`=>16n=36-20=16`
`=>n=16:16=1`
`b)460+85×4=(n+200):4`
`=>460+340=(n+200):4`
`=>800=(n+200):4`
`=>n+200=800×4=3200`
`=>n=3200-200=3000`
`c) 4×120+[45:9+15×(n+1)]=500`
`=>480+5+15(n+1)=500`
`=>15(n+1)=500-480-5=15`
`=>n+1=15:15=1`
`=>n=1-1=0`
`d) 5²: 2n+1`
`=>2n+1 in Ư(25)={+-1,+-5,+-25}`
`=>2n in {0,-2,6,-4,24,-26}`
`=> n in {0,-1,3,-2,12,-13}`