a
\(\frac{\mu_{ A }}{\mu_{ B }}=\frac{ c / V _{ A }}{ c / V _{ B }}=\frac{ V _{ B }}{ V _{ A }}=\frac{1}{2}\)

Let the thickness is \(d\)

\(\frac{d}{v_{B}}-\frac{d}{v_{A}}=5 \times 10^{-10}\)

\(d =\frac{5 \times 10^{-10} \times v _{ A } v _{ B }}{ v _{ A }- v _{ B }}\)

As \(v _{ A }=2 v _{ B } \Rightarrow d =5 \times 10^{-10} \times 2 v _{ B }\)

Or \(d =5 \times 10^{-10} \times v_{ A }\)