The learning curve for a product at XYZ Inc. is $80\%.$ The first unit took $100$ hours to complete. XYZ Inc. wants to determine how many hours the fifth unit will take by using the logarithmic method. The coefficient b for that calculation is approximately:

Question $23$ options:

$\mathrm{.}80$

$-\mathrm{.}2231$

$1\mathrm{.}903$

$-\mathrm{.}0969$

$-\mathrm{.}3219$