Past competition questions and solutions are available online via Parabola Magazine. Below are some links to the solutions published through the Parabola website.
2019: problems and solutions, competition winners
2018: problems and solutions, competition winners
2017: problems and solutions, competition winners
2016: problems and solutions, competition winners
2015: problems and solutions, competition winners
2014: problems and solutions, competition winners
2013: problems and solutions, competition winners
2012: problems and solutions, competition winners