In the figure, two light pulses are sent through layers of plastic with thicknesses of either $L$ or $2L$ as shown and indexes of refraction $n_1=1\mathrm{.}63,n_2=1\mathrm{.}76,n_3=1\mathrm{.}65,n_4=1\mathrm{.}52,n_5=1\mathrm{.}67,n_6=1\mathrm{.}72,$ and $n_7=1\mathrm{.}54.($a$)$ Which pulse travels through the plastic in greater time? $($b$)$ What multiple of $\frac{L}{c}$ gives the difference in the traversal times of the pulses?

$($a$)$ Number

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$($b$)$ Number $$ Units $$