We can start by setting up a proportion using the scale given:
4 cm : 11 ft = 15 cm : x
where x is the length of the actual billboard in feet.
To solve for x, we can cross-multiply and simplify:
4 cm * x = 11 ft * 15 cm
4 cm * x = 165 cm
x = 165 cm / 4 cm/ft
x = 41.25 ft
Therefore, the actual length of the billboard is 41.25 feet.