# Trouble with gambling function

I am trying to make a function which gives us a 1/7 chance of doubling up, or a 6/7 chance of losing $20. Each time the function runs should be an independent event. I have modeled it as follows: f:= RandomChoice[{1/7, 6/7} -> {Function[b, 2b], Function[b, b -20]}](* 1/7 chance of doubling up; 6/7 of losing$20 *)
trialRun1 = NestList[f, 200,5]
trialRun2 = NestList[f, 200,5]
trialRun3 = NestList[f, 200,5]
trialRun4 = NestList[f, 200,5]
trialRun5 = NestList[f, 200,5]


which yields

Out[231]= {200,400,800,1600,3200,6400}

Out[232]= {200,400,800,1600,3200,6400}

Out[233]= {200,180,160,140,120,100}

Out[234]= {200,180,160,140,120,100}

Out[235]= {200,180,160,140,120,100}


As you can see, this is incorrect. f has a 1/7 chance of being the doubling funciton, and a 6/7 chance of being the "lose \$20 function". The events are dependent. How to fix?

f := RandomChoice[{1/7, 6/7} -> {2 #, # - 20}] &

trialRun1 = NestList[f, 200, 5]
trialRun2 = NestList[f, 200, 5]
trialRun3 = NestList[f, 200, 5]
trialRun4 = NestList[f, 200, 5]
trialRun5 = NestList[f, 200, 5]

(* {200, 400, 380, 360, 340, 320}

{200, 180, 160, 140, 120, 100}

{200, 180, 160, 320, 640, 1280}

{200, 180, 160, 320, 300, 280}

{200, 180, 360, 340, 320, 300} *)