For five places you know the magnetic variation in a given year and how much it changes each year. Work out the variation in 2017. Places: Maine (2011, 17°30'W, changing 7' east a year), Possession Sound (2009, 17°15'E, 11' west a year), Fort Lauderdale (2014, 6°30'W, 6' west a year), Pavlof Bay (2014, 12°30'E, 12' west a year), Boothbay (2012, 16°00'W, 5' east a year).
Solution worked out by us from the course material: to be checked by the instructor.