It took me a few minutes to figure this out:
By the time you have found your x5 great grandparents (around 150 years ago) you will potentially have a list of 148 ancestors – and of course it doubles up with every further generation! [Link]OK, barring incestuous liasons between cousins, each of us has 128 "x5 great grandparents." Adding in all the intervening generations gives us 254 ancestors. Subtract the 106 ancestors whose prison records and gambling debts make us ashamed and we arrive at ... 148!