If the learning curve is $80\%$ and the average time to complete units $1$ thru $4$ is $5\mathrm{.}9$ hours, what would be the average time per unit for completing units $4$ thru $8($one doubling$)?$

a$)4\mathrm{.}720$

b$)7\mathrm{.}375$

c$)3\mathrm{.}776$