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Answer:
Let's see
[tex]\\ \rm\rightarrowtail 7x+10+3x-20=90[/tex]
[tex]\\ \rm\rightarrowtail 10x-10=90[/tex]
[tex]\\ \rm\rightarrowtail 10x=100[/tex]
[tex]\\ \rm\rightarrowtail x=10[/tex]