At a learning curve rate of $80\%,$ estimate the time in minutes required to assemble production unit number $100$ given that first unit required $20\mathrm{.}0$ minutes to assemble.

a$.16\mathrm{.}0$

b$.20\mathrm{.}0$

c$.11\mathrm{.}5$

d$.4\mathrm{.}5$