The chance to get piece of photo calculated by the probability theory. You may explore the chance to get a piece of a photo on the main page of every mission in the "Reward for completion" row.
A piece of photo can either drop when you complete a mission for the first time or the 20th time.
The simple explanation of this system:
Imagine a box behind you and with 10 balls in it: 8 black and 2 white. You have to pick a white ball blindly. With each new attempt the amount of balls remains the same.
This is the "20% chance"
For the 40% chance there will be 10 balls in the box, 6 - black, 4 - white.
For the 60% chance there will be 10 balls in the box, 4 - black, 6 - white.
For the 80% chance there will be 10 balls in the box, 2 - black, 8 - white.
Dear players! We sincerely hope this made the chance rate system more clear.
Regards, Nexters Support Team