Since $\triangle IST \sim \triangle NBU$, we have:
$\frac{IS}{NB} = \frac{ST}{BU} = \frac{IT}{NU}$.
We are given that $IS = 12$, $ST = 15$, and $NU = 20$. So we can find the ratio:
$\frac{12}{NB} = \frac{15}{20} = \frac{3}{4}$.
Now we can find the length of
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