Answer
$a.\displaystyle \quad\frac{100W}{L}$
$b.\quad $round skull
Work Step by Step
$a.$
$\displaystyle \frac{L+60W}{L}-\frac{L-40W}{L}=$
To add/subtract rational expressions with the same denominator,
add/subtract numerators and place the sum/difference over the common denominator.
= $\displaystyle \frac{L+60W-(L-40W)}{L}$
= $\displaystyle \frac{L+60W-L+40W}{L}$
= $\displaystyle \frac{100W}{L}$
$b.$
With W=5 and L=6, we evaluate
$\displaystyle \frac{100\times 5}{6}=\frac{500}{6}=83\frac{1}{3}.$
This is over 80, the skull is round.