Answer:
$3166.67 for exactly twice as many, more if it is more than twice as many.
Step-by-step explanation:
The average will always be between between the two numbers, and since there are more full time lecturers the average will be closer to that value, so the higher value. This is especially true if there are twice as many or more. The more there are as a difference between the two the closer it will be to the one that has more.
Let's say there are x part time and y full time. the average works out to be (x*25,000 + y*35000)/(x+y). Now, if there are exactly twice as many full time instead of part time then you can put y = 2x and you can solve for a number.
(x*25,000 + y*35000)/(x+y)
(x*25,000 + 2x*35000)/(x+2x)
95,000x/3x
3166.67
So indeed, this is closer to 35000 than 25000, and also is the minimum for the first part since this only gets higher if there are more than twice as many full time than part time.