Since Patricia has 6 dollars, then Lisa has 6 * 5 = <<6*5=30>>30 dollars.
Patricia’s money is double Charlotte’s, meaning that Charlotte has 30/2 = <<30/2=15>>15 dollars.
Adding up all the money they have gives a total of 6 + 30 + 15 = <<6+30+15=51>>51 dollars.
Since the rare card costs 100 dollars, the girls require an additional 100 -51 = <<100-51=49>>49 dollars
#### 49