First subtract the 300 MB Elsa spent on YouTube: 500 MB - 300 MB = <<500-300=200>>200 MB.
Then multiply 200 MB by 2/5th to find out how much data Elsa spent on Facebook: 200 MB * 2/5 = <<200*2/5=80>>80 MB.
Finally, subtract the data Elsa spent on Facebook from the 200 MB: 200 MB - 80 MB = <<200-80=120>>120 MB.
#### 120