(b) Thickness$\ d = \frac{V}{A} = \frac{m}{\rho A} = \frac{zlt}{\rho A}$ $= \frac{\left( 3.3 \times 10^{7} \right) \times (1.5) \times (20 \times 60)}{9000 \times (50 \times 10 \times 2) \times 10^{- 4}} = 6.6 \times 10^{- 6}$ m.
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Ph.D. in Physics with 19+ years of teaching experience.