Answer
$$F\approx16.1 N$$
Work Step by Step
A penguin sled without any additional force moves down, so friction force will parallel to the plane and up.
$$\sum F_{y}=F_N-mgcos(\theta)\\
F_N=mgcos(\theta)\\
\sum F_{x}=F+f_k-mgsin(\theta)\\
\sum F_{x}=F+mgcos(\theta)\mu_k-mgsin(\theta)=0\\
F=mg(sin(\theta)-\mu_kcos(\theta))\\
F=80(sin(20^{\circ})-0.15cos(20^{\circ}))\approx16.1N$$
