(a) By using formula, $\frac{\mu ₂}{v} - \frac{\mu ₁}{u} = \frac{\mu ₂\ - \mu ₁}{R}$ $\Longrightarrow \frac{1.5}{v} - \frac{1}{( - 15)} = \frac{1.5 - 1}{+ 30}$ $\Longrightarrow v = \ - 30\ cm$
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