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No. 1218
Total cost to reach level X is: sum( 10 * i, i = 1..X) We can factor out the 10, since it does not depend on i. 10 * sum(i, i = 1..X) The sum of consecutive integers is well known, and can be found with some googling if you forget it (like I do). 10 * ( X * (X + 1) / 2 ) Simplify. 5 * (X^2 + X) HTH!
