Question

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 = negative StartFraction 1 Over 20 EndFraction x minus 5
y = negative StartFraction 1 Over 20 EndFraction x + 5
y = negative 20 x minus 5
y = negative 20 x + 5

Answer 1

y = -1/20 x + 5

Reason: the dune loses 1 foot every 20 years, so the rate is -1/20 foot per year, and at x = 0 the height is 5, giving y = 5 - (1/20)x = -1/20 x + 5.