Answer
$t=5$:
$N\approx333~deers$
$t=10$:
$N=500~deers$
$t=25$:
$N=800~deers$
Work Step by Step
$t=5$:
$N=\frac{20(5+3(5))}{1+0.04(5)}=\frac{20(20)}{1.2}=333.33\approx333$
$t=10$:
$N=\frac{20(5+3(10))}{1+0.04(10)}=\frac{20(35)}{1.4}=500$
$t=25$:
$N=\frac{20(5+3(25))}{1+0.04(25)}=\frac{20(80)}{2}=800$