Science and Technology have completely changed human life.

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1) ta có: x+(x+1)+(x+2)+...+(x+30)=1240

=>(x+x+x+...+x)+(1+2+3+...+30)=1240

=>31x+[(30+1).30:2]=1240

=>31x+465=1240

=>31x=1240-465=775

==>x=775:31=25