$4-(20$ points$)$ A small machine shop, with $30hp$ of connected load, purchases electricity under the following monthly rates $($assume any demand charge is included in this schedule$)$:

First $50kw-$hr per hp of connected load at $8\mathrm{.}5$ cents per kw$-$hr

Next $50kw-$hr per hp of connected load at $6\mathrm{.}5$ cents per kw$-$hr

Next $200kw-$hr per hp of connected load at $5\mathrm{.}0$ cents per kw$-$hr

All electricity over $250kw-$hr per hp of connected load at $3\mathrm{.}7$ cents per kw$-$hr The shop uses $2800kw-$hr per month.