$4\mathrm{.}49$ The following is a schematic micrograph that represents the microstructure of some hypothetical metal.

$\backslash (50\backslash$mu $\backslash$mathrm$\{$~m$\}\backslash )$

Determine the following:

$($a$)$ Mean intercept length

$($b$)$ ASTM grain size number, $\backslash ($ G $\backslash )$