MATHS!! help[~

The resistance that a raft experiences when it travels in water varies directly as the cube of its speed.
a) If the speed of the raft double , find the ratio of the new resistance to the original resistance.
let r be the resistance,s be its speed,
r = ks^3,where k is constant
now,at initial,
r[0] = k(s[0])^3
if the speed is double,
r[1] = k(2s[0])^3
r[1] = 8k(s[0])^3
so,r[1] : r[0] = 8 : 1
b) If the water resistance increases by 72.8% , what is the percentage incentage increase in the speed of the raft?
now,
r = ks^3
s^3 = r/k
(s[0])^3 = r[0]/k
s[0] = (r[0]/k)^1/3
when r increases ny 72.8%
(s[1])^3 = (1 + 0.728)r[0]/k
(s[1])^3 = 1.728r[0]/k
s[1] = (1.728r[0]/k)^1/3
s[1] = 1.2(r[0]/k0^1/3
the increase percentage:
{(s[1] - s[0]) / s[0]} x 100%
= (1.2 - 1) x 100%
= 20%



Answer: a) 8 : 1 ;
b) 20%