u/Background-Pop-1685

Okay so I have a hypothetical question. Consider a 100 bp DNA sequence coding for a RNA. If the whole DNA is transcribed completely then what is the probability of stop codon in it? Actually someone asked me this question and I wasn't sure what exactly to answer. I thought like there are total of 64 codons and 3 of them are stop codons. So shouldn't the probability be 3/64? Should the number of base pairs matter? The number of stop codons may change according to the number of base pairs but should the probability be affected?