$($b$)$ In the re$-$development of the Kai Tak airport, a fill embankment of volume $60,000m^3$ and of void ratio of $0\mathrm{.}75$ is to be built. To get the fill soil materials, two potential "borrow pits" were identified: $(1)$ Tsuen Wan site: in$-$situ void ratio is $0\mathrm{.}8$ and cost of transport is HK $$\frac{100}{m^3}$; $(2)$ Ma On Shan site: in$-$situ void ratio is $1\mathrm{.}5$ and cost of transport is $HK$$$\frac{70}{m^3}.$ Which borrow pit should be used and what is the total cost?