input: Write a python function to create a bank account class with deposit and withdraw methods.
  assert bank.deposit(100) == 100
  assert bank.withdraw(50) == 50
  assert bank.deposit(200) == 250
output: class BankAccount:
      def __init__(self):
          self.balance = 0
  
      def deposit(self, amount):
          self.balance += amount
          return self.balance
  
      def withdraw(self, amount):
          if amount <= self.balance:
              self.balance -= amount
              return self.balance
          else:
              raise ValueError("Insufficient funds")
  
  # Usage
  bank = BankAccount()
  assert bank.deposit(100) == 100
  assert bank.withdraw(50) == 50
  assert bank.deposit(200) == 250