jmc

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 $50÷2=\boxed{25}.$