The Mind Sports Olympiad holds an annual Mental calculations contest since 1997.
The 2018 contest happened last month in London. Here is the contest’s questions paper:
https://worldmentalcalculation.com/wp-content/uploads/2018/09/MSO-MCWC-paper-2018.pdf (27 pages tasksheet). Participants had 3.5 hours to solve all these of questions, just by writing the final answer down.
The solutions in the paper above, are shown in the right section, underlined in bold. Next to each solution you can see the number of respective points that each correct question was awarded.
The final results can be found here: https://worldmentalcalculation.com/2018/08/23/mind-sports-olympiad-results-2018
16-year-old Wenzel Grüß from Germany won the contest with 3329 correct points out of the maximum score of 4040 possible correct points. He solved 82.4 % of the maximum points.