其它
Lucy writes numbers from $1$ to $200$ on the blackboard. Then, she plays a game with her friend Mike. Each time they take turn to delete the first two numbers in the sequence and write their sum by the end of the sequence. For example, Lucy deletes $1$ and $2$, then writes $3$ behind $200$ for the first time. They play this game until there is only one number on the blackboard. What is the number? 
Overseas Competition->Knowledge Point->Calculation Modules->Sequences and Number Tables->Arithmetic Sequences
The result is equal to the sum of all numbers from $1$ to $200$, which is $(1+200)\times200\div2=20100$. 