Gas and transaction fees in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Calculate total transaction cost in Ethereum

Given the following transaction parameters, what is the total cost in Ether for this transaction?

Gas limit: 21000
Gas price: 50 Gwei (1 Gwei = 10^-9 Ether)

Blockchain / Solidity

gas_limit = 21000
gas_price_gwei = 50
gas_price_ether = gas_price_gwei * 1e-9
total_cost = gas_limit * gas_price_ether
print(total_cost)

A 0.0000105

B 0.00105

C 0.000105

D 0.0105

💡 Hint

Multiply gas limit by gas price converted to Ether.

Gas price in Ether = 50 * 10^-9 = 0.00000005 Ether.
Total cost = 21000 * 0.00000005 = 0.00105 Ether.

🧠 Conceptual

intermediate

Understanding Gas Limit and Gas Price

Which statement correctly describes the relationship between gas limit and gas price in a blockchain transaction?

A Gas limit and gas price are the same and used interchangeably.

B Gas price is the maximum gas a transaction can use; gas limit is the amount paid per unit of gas.

C Gas limit is the maximum amount of gas a transaction can use; gas price is the amount paid per unit of gas.

D Gas limit is the total fee paid; gas price is the transaction size.

💡 Hint

Think about how much gas you allow and how much you pay per gas unit.

Gas limit sets the max gas units a transaction can consume.
Gas price is the cost per gas unit in Ether.

🔧 Debug

advanced

Identify the error in gas fee calculation code

What error does the following code produce when calculating transaction fees?

gas_limit = 30000
gas_price = '20'
total_fee = gas_limit * gas_price
print(total_fee)

A No error, outputs '20' repeated 30000 times

B TypeError: can't multiply sequence by non-int of type 'str'

C TypeError: unsupported operand type(s) for *: 'int' and 'str'

D TypeError: can't multiply sequence by non-int of type 'int'

💡 Hint

Check the data types of variables before multiplication.

No TypeError occurs because in Python, multiplying an int by a str repeats the string instead of performing numeric multiplication. The output is '20' repeated 30,000 times.

❓ Predict Output

advanced

Calculate transaction fee with dynamic gas price

What is the output of this code snippet?

def calculate_fee(gas_limit, gas_price_gwei):
    gas_price_ether = gas_price_gwei * 1e-9
    return gas_limit * gas_price_ether

fee = calculate_fee(50000, 100)
print(f"{fee:.6f}")

A 0.005000

B 0.000500

C 0.050000

D 0.000050

💡 Hint

Multiply gas limit by gas price converted to Ether.

Gas price in Ether = 100 * 10^-9 = 0.0000001 Ether.
Total fee = 50000 * 0.0000001 = 0.005 Ether.

🧠 Conceptual

expert

Why do blockchain transactions require gas fees?

Which is the main reason blockchain networks require gas fees for transactions?

A To increase the value of the cryptocurrency by burning tokens.

B To limit the number of transactions by setting a fixed fee per transaction.

C To pay miners directly for storing user data permanently.

D To prevent spam and allocate network resources fairly by charging for computation.

💡 Hint

Think about why networks charge fees for running code.

Gas fees prevent spam and ensure users pay for the computational work their transactions require, helping allocate resources fairly.