## Calculus (3rd Edition)

$157$
The number of gorillas is given as follows $$2 \pi \int_{0}^{5} r \rho(r) d r=\int_{0}^{5} \frac{104 \pi r}{\left(1+r^{2}\right)^{2}}dr\\ =\int_{0}^{5} 104 \pi r \left(1+r^{2}\right)^{-2}dr=-\left.\frac{52 \pi}{1+r^{2}}\right|_{0} ^{5}=50 \pi\approx 157.$$