the power P (measured in horsepower, hp) is needed to propel a boat is directly proportional to the cube of the speed s thus

P = k*s^3

where k is constant

an 80-hp engine is needed to propel a boat at 10 knots

80 = k*10^3

k = 80/10^3

k = 0.08

P = 0.08*s^3

find the power needed to drive the boat at 15 knots that is for s = 15 knots we need to find P using the above formula

P = 0.08*15^3

P = 270 hp

the power needed to drive the boat at 15 knots is 270 hp.