`2,` `9,9×y+y:10=0,1`
`9,9×y+0,1×y=0,1`
`y×(9,9+0,1)=0,1`
`y×10=0,1`
`y=0,1×10`
`y=1`
`3,` `A=20-20,2+20,7-21,2+21,7-22,2+22,7-23,2+23,7-24,2+24,7-25,2+25,7`
`A=20+20,7-20,2+21,7-21,2+22,7-22,2+23,7-23,2+24,7-24,2+25,7-25,2`
`A=20+(20,7-20,2)+(21,7-21,2)+(22,7-22,2)+(23,7-23,2)+(24,7-24,2)+(25,7-25,2)`
`A=20+0,5+0,5+0,5+0,5+0,5+0,5` `A=20+0,5×6`
`A=20+3`
`A=23`