Giải giúp toán lớp 5 ạ

Câu `10` `:`
`a,` `(` `1` `+` `2` `+` `3` `+` `...` `+` `99` `)` `xx` `(` `25,4` `xx` `8,2` `-` `4,1` `xx` `50,8` `)`
`=` `(` `1` `+` `2` `+` `3` `+` `...` `+` `99` `)` `xx` `(` `208,28` `-` `208,28` `)`
`=`  `(` `1` `+` `2` `+` `3` `+` `...` `+` `99` `)` `xx` `0`
`=` `0`
`b,` `8,125` `xx` `10` `-` `81,25` `:` `0,2` `-` `81,25` `:` `0,25`
`=` `81,25` `-` `81,25` `xx` `5` `-` `81,25` `xx` `4`
`=` `81,25` `xx` `(` `1` `-` `5` `-` `4` `)`
`=` `81,25` `xx` `-8`
`=` `-650`
`\ttcolor{blonde}{@C}` `\ttcolor{blonde}{a}` `\ttcolor{blonde}{p}` `\ttcolor{blonde}{t}` `\ttcolor{blonde}{a}` `\ttcolor{blonde}{i}` `\ttcolor{blonde}{n}` `\ttcolor{blonde}{B}` `\ttcolor{blonde}{o}` `\ttcolor{blonde}{y}`