Gas and transaction fees in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the gas limit for a transaction.

Blockchain / Solidity

transaction = {"to": recipient, "value": amount, "gas": [1]

Drag options to blanks, or click blank then click option'

A5000

B21000

C100000

D30000

Attempts:

3 left

💡 Hint

Common Mistakes

Using too low or too high gas limits causing transaction failure or waste.

The standard gas limit for a simple ETH transfer is 21000 units.

2fill in blank

medium

Complete the code to calculate the total transaction fee.

Blockchain / Solidity

total_fee = gas_price * [1]

Drag options to blanks, or click blank then click option'

Avalue

Bnonce

Cblock_number

Dgas_limit

Attempts:

3 left

💡 Hint

Common Mistakes

Multiplying gas price by nonce or value instead of gas limit.

The total fee is gas price multiplied by gas limit.

3fill in blank

hard

Fix the error in the code to set the gas price correctly.

Blockchain / Solidity

transaction['gasPrice'] = [1]

Drag options to blanks, or click blank then click option'

Agas_price

Bgasprice

CGasPrice

DgasPrice

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong case or misspelled variable names.

The correct variable name is gas_price matching the code context.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension for gas fees above a threshold.

Blockchain / Solidity

high_fees = {tx_id: tx['gasPrice'] * [1] for tx_id, tx in transactions.items() if tx['gasPrice'] [2] threshold}

Drag options to blanks, or click blank then click option'

Atx['gasLimit']

Btx['value']

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong keys or wrong comparison operators.

Calculate fee by multiplying gasPrice and gasLimit, filter fees greater than threshold.

5fill in blank

hard

Fill all three blanks to create a filtered dictionary of transactions with fees above a limit.

Blockchain / Solidity

filtered = {tx['from']: tx['gasPrice'] * [1] for tx in tx_list if tx['gasPrice'] [2] min_price and tx['gasLimit'] [3] max_limit}

Drag options to blanks, or click blank then click option'

Atx['gasLimit']

B>

C<

Dtx['value']

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up comparison operators or using wrong keys.

Calculate fee by gasPrice * gasLimit, filter gasPrice greater than min_price and gasLimit less than max_limit.

Practice

1. What does gas represent in a blockchain transaction?

easy

A. The amount of computational work required to process the transaction

B. The total amount of cryptocurrency sent in the transaction

C. The number of transactions in a block

D. The time it takes to confirm a transaction

Solution

1. Step 1: Understand the role of gas in blockchain

   Gas is a unit that measures how much computational effort is needed to execute operations like transactions or smart contracts.

2. Step 2: Differentiate gas from other transaction elements

   Gas is not the amount of cryptocurrency sent, nor the number of transactions or confirmation time.

3. Final Answer:

   The amount of computational work required to process the transaction -> Option A

4. Quick Check:

   Gas = computational work [OK]

Hint: Gas measures work, not money or time [OK]

Common Mistakes:

• Confusing gas with transaction value
• Thinking gas is transaction speed
• Mixing gas with block size

2. Which of the following is the correct formula to calculate transaction fees?

easy

A. Transaction fees = gas price ÷ gas used

B. Transaction fees = gas price - gas used

C. Transaction fees = gas used + gas price

D. Transaction fees = gas used x gas price

Solution

1. Step 1: Recall the transaction fee formula

   Transaction fees are calculated by multiplying the gas used by the gas price.

2. Step 2: Verify the formula correctness

   Dividing, adding, or subtracting gas used and gas price does not represent the fee calculation.

3. Final Answer:

   Transaction fees = gas used x gas price -> Option D

4. Quick Check:

   Fees = gas used x gas price [OK]

Hint: Multiply gas used by gas price for fees [OK]

Common Mistakes:

• Using division instead of multiplication
• Adding gas used and gas price
• Subtracting gas values

3. Given a transaction that used 21000 gas units and a gas price of 50 gwei, what is the total transaction fee in gwei?

medium

A. 21000 gwei

B. 1,050,000 gwei

C. 420 gwei

D. 50 gwei

Solution

1. Step 1: Identify given values

   Gas used = 21000 units, Gas price = 50 gwei per unit.

2. Step 2: Calculate total fee

   Total fee = gas used x gas price = 21000 x 50 = 1,050,000 gwei.

3. Final Answer:

   1,050,000 gwei -> Option B

4. Quick Check:

   21000 x 50 = 1,050,000 [OK]

Hint: Multiply gas used by gas price directly [OK]

Common Mistakes:

• Multiplying incorrectly
• Confusing gas units with gwei
• Using gas price as total fee

4. Identify the error in this transaction fee calculation code snippet:

gas_used = 30000
gas_price = 20
transaction_fee = gas_used + gas_price
print(transaction_fee)

medium

A. Using addition instead of multiplication for fee calculation

B. Incorrect variable names

C. Missing print statement

D. Gas price should be divided by gas used

Solution

1. Step 1: Review the code logic

   The code adds gas_used and gas_price instead of multiplying them.

2. Step 2: Understand correct fee calculation

   Transaction fee should be gas_used x gas_price, not addition.

3. Final Answer:

   Using addition instead of multiplication for fee calculation -> Option A

4. Quick Check:

   Fee = gas_used x gas_price, not + [OK]

Hint: Fees multiply gas used by gas price, not add [OK]

Common Mistakes:

• Adding gas values instead of multiplying
• Confusing variable names
• Ignoring correct formula

5. A user wants to speed up their transaction by increasing the gas price from 40 gwei to 60 gwei. If the gas used remains 21000, how much more will the user pay in transaction fees compared to before?

hard

A. 600,000 gwei more

B. 1,260,000 gwei more

C. 420,000 gwei more

D. 840,000 gwei more

Solution

1. Step 1: Calculate original fee

   Original fee = 21000 x 40 = 840,000 gwei.

2. Step 2: Calculate new fee

   New fee = 21000 x 60 = 1,260,000 gwei.

3. Step 3: Find difference

   Difference = 1,260,000 - 840,000 = 420,000 gwei more.

4. Final Answer:

   420,000 gwei more -> Option C

5. Quick Check:

   Increase = (60-40)x21000 = 420,000 [OK]

Hint: Subtract old fee from new fee to find extra cost [OK]

Common Mistakes:

• Calculating total fee instead of difference
• Mixing gas price and gas used
• Incorrect multiplication