Something I saw on Reddit and decided to give a try. Link in header.

string_probability_challenge.py

# Coding Challenge 
# Reddit: https://www.reddit.com/r/learnprogramming/comments/847ckg/i_failed_a_coding_interview_but_then_afterwards/
# The problem was given a file of sentences, one sentence per line, 
# implement a function that takes in 2 words, prob(a, b), 
# and outputs the probability of a occurring given that the 
# preceding word is b. 

# Some test data already in a list with punctuation removed
sentences = ["Please buy me a drink", 
"Please buy me a drink Bob",
"Please get me a drink Sue",
"Please find me a drink Joe",
"Please give me a Coke Joe"
]

def prob(a, b, sentences):
	"""Probability of a occurring given that b is the preceding 
	word in the supplied sentences"""
	
	# Text stores all text
	text = ""
	
	# Could use join but let's use a loop
	for sentence in sentences:
		text += " " + sentence.lower() 
	
	# Create a list of all the words	
	words = text.split(" ")

	# Keep track of how many a and b there are
	a_count = 0
	b_count = 0

	# Iterate through the list and find the word b
	for index in range(len(words)-1):
		if words[index] == b.lower():
			b_count += 1
			# If the next word is a
			if words[index+1] == a.lower():
				a_count += 1
					
	# Calculate the probability
	if b_count != 0:
		probability = float(a_count) / float(b_count)
	else:
		probability = "{} not found".format(b)
	
	# Show result
	print("{}: {}  {}: {}  Probability: {}".format(a, a_count, b, b_count, probability))

print("\n\nTest Results\n")
prob("drink", "a", sentences)
prob("me", "drink", sentences)
prob("buy", "Please", sentences)
prob("get", "cheer", sentences)
print("\n\n")