The second student guessed 800 because 8 x 100 = <<800=800>>800
The third guessed 600 because 800 - 200 = <<800-200=600>>600
The total guesses of the first three is 1500 because 100 + 800 + 600 = <<100+800+600=1500>>1500
The average of the first three guesses is 500 because 1500 / 3 = <<1500/3=500>>500
The fourth student guessed 525 because 500 + 25 = <<500+25=525>>525
#### 525