Answer :

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x/3+10=15
     -10   -10
x/3= 5
x= 3*5
x=15

check by substitution 15/3 +10= 15
                                  5+10=15, this is true 
Therefore x=15 is the answer
[tex] \frac{x}{3}+10=15 \hbox{ /|10 (substract 10 from both) } [/tex]

[tex] \frac{x}{3}=15-10=5 [/tex] ⇒ [tex]x=3*5 [/tex]

[tex]\boxed {x=15}[/tex]