Try a Simpler Version #5

There were four Thanksgiving turkeys in a row.
The average of the weights of the first two turkeys was 15 pounds.
The average of the weights of the middle two turkeys was 18.3 pounds.
The average of the weights of the last two turkeys was 15.4 pounds.
Based on this information, what was the average of the weights of the first and last turkeys?

As usual, a diagram will help.
An important feature of this problem, that you must take into account, is that it is not possible to determine the weights of the four turkeys.
But it is possible to answer the question. You can do it either by algebra, or by guessing and checking, or by assuming a specific weight for one of the turkeys, and then calculating the weights of the others based on that. (Of course, a complete solution of this last sort should include an explanation of why the choice of the weight of the first turkey doesn't affect the answer to the problem).
As always, be sure to explain the work you did to arrive at your answers!