Four, (10 points) Person A and Person B process a batch of parts. If A and B work together for 6 days, and then A continues alone for 5 more days, the task can be completed; if A and B work together for 7 days, and then B continues alone for 5 more days, the task can also be completed. Now, if A first processes 300 parts, and then the two work together for 8 days to complete the task. How many parts are there in total?
Four, suppose there are $s$ parts in total, person A completes $x$ parts per day, and person B completes $y$ parts per day. According to the problem, we have
$$
\left(\begin{array}{l}
6 x+6 y+5 x=s, \\
7 x+7 y+5 y=s, \\
300+8(x+y)=s .
\end{array}\right.
$$

Solving this, we get $s=2700$ ( $\uparrow$ ).