Let $t$ be TNYWR.

If $\frac{5+x}{t+x}=\frac{2}{3}$, what is the value of $x$ ?

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Multiplying through by $3(t+x)$ gives $3(5+x)=2(t+x)$.

Expanding gives $15+3 x=2 t+2 x$ which can be rearranged to $x=2 t-15$.

Substituting $t=13$ gives $x=2(13)-15=11$.