OK, bet someone any amount you want to take off them, that you can speak for sixty seconds without hesitation or repetition, and you will not use any word with the letter 'A' in it.
After 99 :
you could go into negatives (minus1., minus2...) but that could be repetition of 'minus'
or hundreds (one hundred,... nineteen hundred...) or millions (one million,... 99 million) then billions... but maybe that's repeating hundreds, millions, billions
Or do fractions (fifth, sixth, seventh, eighth...)
Thanks for the responses - including the diversions from the main idea.
As fans of Jack Reacher will know, you simply count from one to a hundred slowly, which will take way more than sixty seconds.
The people I have tried it on smiled when I got to about 'six' and they realised what was happening - then I simply stopped and confirmed that you can go up to one hundred without using the letter 'A' or repeating yourself.