This video mainly talks about Gauss sums. The formula to find the sum of a sequence is n(n + 1) / 2, where n is the length of the sequence.
This video covers the concept of letter substitution, which comes up often in elementary competition math problems. Remember to use addition, subtraction, multiplication, and division rules while solving these types of problems. They will help you narrow down what each letter stands for.
This video contains some competition-level problems of letter substitution. Again, use your operation rules to narrow down what each letter stands for. We highly recommend practicing this topic further, as it is almost guaranteed to come up in elementary contests.