Explanation:

calculate the time it should take for the third unit with an $80$ percent learning rate

Where:

$-$

is the time for the nth unit.

$-$

is the time for the first unit $(10$ hours in this case$).$

$-$

is the learning rate $(80$ percent, so $0\mathrm{.}8).$

$-$

is the unit number $(3$ in this case$).$