其它
Four numbers are written in a row. The average of the first two is $21$ , the average of the middle two is $26$ , and the average of the last two is $30$ . What is the average of the first and last of the numbers? (2022 AMC 8 Problems, Question \#16) 
Overseas Competition->Knowledge Point->Calculation Modules
Note that the sum of the first two numbers is $21 \cdot 2=42$, the sum of the middle two numbers is $26 \cdot 2=52$, and the sum of the last two numbers is $30 \cdot 2=60$.  It follows that the sum of the four numbers is $42+60=102$, so the sum of the first and last numbers is $102-52=50$. Therefore, the average of the first and last numbers is $50 \div 2=$ (B) $25$. 