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original_string = "sentence one. sentence two. sentence three"
last_char_index = original_string.rfind(".")
new_string = original_string[:last_char_index] + "," + original_string
print(new_string)
"sentence one. sententce two, sentence three"
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string = "Hello, World!"
character = "o"
last_occurrence = string.rfind(character)
print(last_occurrence)