Answer
$(a)$ $P=ks^3$
$(b)$ $k=0.08$
$(c)$ $P=270 hp$
Work Step by Step
$(a)$ According to the problem, we have $P=ks^3$
Where $k$ is the constant of proportionality.
$(b)$
$P=80 hp$
$s=10 knots$
$k$ is unknown
$80=k\times10^3$
$1000k=80$
$k=0.08$
$(c)$
$s=15 knots$
$P$ is unknown
$P=0.08\times15^3 = 0.08\times3375=270 hp$
