If you took 100 photographs of the project 18 hours ago, and today 20% few photographs have been taken, then 20/100*100 = 20 fewer photographs of the project have been taken today.
The total number of photographs of the project that have been taken today is 100-20 = <<100-20=80>>80
So far, you've taken 80+100 = <<80+100=180>>180 photographs of the project.
Since the target number of photographs is 300, the number of photographs that you need to take to reach the target is 300-180 = <<300-180=120>>120
#### 120