A sand dune stands 5 feet above sea level. The hill is eroding at a rate of 1 foot per 20 years. Let y represent the height of the sand dune after x years. Which equation represents the situation?

Y at initial time is 5 and therefore 5 is a constant

x is the amount of years so it is one foot per 20 years therefore (1/20) x or (x/20)

The height of the sand dune at any time is 5 foot - (x/20)

therefore

y = 5 - (x/20)