Just by using basic algebra you can find the answer by saying 0.25 ft every year which in a fraction would be 0.25 ft / 1 year = 15 ft / x. Cross multiply so it becomes 0.25x = 15 ft. Divide 15 ft by the 0.25 ft by bringing it to the other side of the equal sign, 15/0.25= 60 years
