Token counting and cost estimation in Prompt Engineering / GenAI - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding token counting in text inputs

If a text input contains the sentence:
"Hello world! How are you today?"
How many tokens would a typical tokenizer count for this sentence?

A 8 tokens

B 6 tokens

C 5 tokens

D 7 tokens

💡 Hint

Think about how punctuation and spaces are usually split into tokens.

Most tokenizers split words and punctuation separately. Here, 'Hello', 'world', '!', 'How', 'are', 'you', 'today', '?' are 8 tokens.

❓ Predict Output

intermediate

Calculate token cost for a prompt

Given a prompt with 150 tokens and a model charging $0.002 per 1,000 tokens, what is the cost for processing this prompt?

A $0.03

B $0.0003

C $0.00015

D $0.003

💡 Hint

Calculate cost by multiplying tokens by price per token.

Cost = (150 tokens / 1000) * $0.002 = 0.15 * 0.002 = $0.0003

❓ Model Choice

advanced

Choosing a model based on token limits

You have a text input of 12,000 tokens. Which model should you choose if Model A supports up to 8,000 tokens and Model B supports up to 16,000 tokens?

A Either model works

B Model A

C Model B

D Neither model works

💡 Hint

Check the maximum token limit for each model.

Model A supports only 8,000 tokens, which is less than 12,000 tokens input. Model B supports 16,000 tokens, which is enough for 12,000 tokens input.

❓ Metrics

advanced

Estimating total cost for multiple API calls

You make 3 API calls with token usage: 500, 1,200, and 2,300 tokens. The cost is $0.0015 per 1,000 tokens. What is the total cost?

A $0.0075

B $0.009

C $0.0069

D $0.006

💡 Hint

Sum tokens first, then multiply by cost per token.

Total tokens = 500 + 1200 + 2300 = 4000 tokens. Cost = (4000 / 1000) * 0.0015 = 4 * 0.0015 = $0.006

🔧 Debug

expert

Identify the error in token cost calculation code

What error does this code produce?

tokens = 1200
price_per_1000 = 0.002
cost = tokens * price_per_1000
print(f"Cost: ${cost}")

Prompt Engineering / GenAI

tokens = 1200
price_per_1000 = 0.002
cost = tokens * price_per_1000
print(f"Cost: ${cost}")

A Prints Cost: $2.4

B Prints Cost: $0.0024

C SyntaxError due to f-string

D TypeError due to division

💡 Hint

Check the order of operations and units in the formula.

The code incorrectly omits the division by 1000. It calculates cost = tokens * price_per_1000 = 1200 * 0.002 = 2.4, printing Cost: $2.4, which is too high (correct would be $0.0024).

Practice

1. What is a token in the context of AI language models?

easy

A. A hardware component

B. A small piece of text like a word or part of a word

C. A programming language

D. A type of AI model

Solution

1. Step 1: Understand token meaning

   Tokens are the smallest pieces of text that AI models read, such as words or parts of words.

2. Step 2: Identify correct definition

   Among the options, only A small piece of text like a word or part of a word correctly describes tokens as small text pieces.

3. Final Answer:

   A small piece of text like a word or part of a word -> Option B

4. Quick Check:

   Token = small text piece [OK]

Hint: Tokens are text chunks, not models or hardware [OK]

Common Mistakes:

• Confusing tokens with AI models
• Thinking tokens are programming languages
• Assuming tokens are hardware parts

2. Which of the following Python code snippets correctly counts tokens using a simple split by spaces?

easy

A. tokens = text.split(' ')
count = len(tokens)

B. tokens = text.count(' ')
count = tokens + 1

C. tokens = len(text)
count = tokens

D. tokens = text.split()
count = tokens

Solution

1. Step 1: Understand token counting by splitting

   Splitting text by spaces returns a list of tokens; counting tokens is length of that list.

2. Step 2: Check each option

   tokens = text.split(' ') count = len(tokens) splits by space and counts tokens correctly. tokens = text.count(' ') count = tokens + 1 counts spaces but needs +1 for tokens. tokens = len(text) count = tokens counts characters, not tokens. tokens = text.split() count = tokens assigns list to count, which is incorrect.

3. Final Answer:

   tokens = text.split(' ') count = len(tokens) -> Option A

4. Quick Check:

   Split by space + len() = token count [OK]

Hint: Use split(' ') and len() to count tokens simply [OK]

Common Mistakes:

• Counting characters instead of tokens
• Forgetting to add 1 when counting spaces
• Assigning list directly to count variable

3. Given the text: "Hello world! This is AI." and a token counting method that splits by spaces, what is the token count?

medium

A. 7

B. 6

C. 4

D. 5

Solution

1. Step 1: Split the text by spaces

   Splitting "Hello world! This is AI." by spaces gives: ['Hello', 'world!', 'This', 'is', 'AI.']

2. Step 2: Count the tokens

   There are 5 tokens in the list.

3. Final Answer:

   5 -> Option D

4. Quick Check:

   5 tokens from splitting by space [OK]

Hint: Count words separated by spaces for quick token count [OK]

Common Mistakes:

• Counting punctuation as separate tokens
• Adding extra tokens incorrectly
• Miscounting spaces

4. You wrote this code to count tokens but it gives an error:

text = "AI is fun"
tokens = text.split
count = len(tokens)

What is the error and how to fix it?

medium

A. Missing parentheses in split method call; fix with text.split()

B. len() cannot be used on list; use count() instead

C. text should be a list, not string

D. split method does not exist for strings

Solution

1. Step 1: Identify the error in method call

   text.split is a method reference, not a call. It needs parentheses to execute.

2. Step 2: Fix the code

   Change text.split to text.split() to get the list of tokens, then len() works correctly.

3. Final Answer:

   Missing parentheses in split method call; fix with text.split() -> Option A

4. Quick Check:

   Use split() with parentheses to call method [OK]

Hint: Always add () to call string methods like split() [OK]

Common Mistakes:

• Forgetting parentheses on method calls
• Using len() on method instead of list
• Thinking split is not a string method

5. You want to estimate the cost of an AI request. The model charges $0.0001 per token. If your input has 120 tokens and output is expected to be 80 tokens, what is the total estimated cost?

hard

A. $0.012

B. $0.01

C. $0.02

D. $0.008

Solution

1. Step 1: Calculate total tokens used

   Total tokens = input tokens + output tokens = 120 + 80 = 200 tokens.

2. Step 2: Multiply total tokens by cost per token

   Cost = 200 tokens * $0.0001 = $0.02.

3. Step 3: Check options carefully

   $0.02 shows $0.02, but $0.012 shows $0.012 which is incorrect. Recalculate carefully: 200 * 0.0001 = 0.02, so $0.02 is correct.

4. Final Answer:

   $0.02 -> Option C

5. Quick Check:

   200 tokens * $0.0001 = $0.02 [OK]

Hint: Add input and output tokens, multiply by cost per token [OK]

Common Mistakes:

• Multiplying only input tokens by cost
• Multiplying only output tokens by cost
• Misreading decimal places in cost