Katrina has 85/5 = <<85/5=17>>17 marbles.
Twice as many marbles as Katrina is 17 x 2 = <<17*2=34>>34
Amanda has 34 -12 = <<34-12=22>>22 marbles.
Mabel has 85 - 22 = <<85-22=63>>63 more marbles than Amanda
#### 63