Since $\triangle IST \sim \triangle NBU$, we have $\frac{IS}{NB} = \frac{ST}{BU} = \frac{IT}{NU}$.
From the given information, we have $IS = 12$, $ST = 16$, $NB = 3x$, and $BU = 4x$. Plugging these values into the equation, we get $\frac{12}{3x} =
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