input: from typing import List
  
  def predict_customer_ltv(transaction_data: List[List[float]]) -> float:
      """ Predict the customer lifetime value based on the past transactions of the customer. 
      The function uses a machine learning model trained on transaction data.
      >>> predict_customer_ltv([[10, 100, 5, ...]])
      150.0
      >>> predict_customer_ltv([[5, 50, 2, ...]])
      90.0
      """
output:     # Placeholder for the machine learning model
      model = lambda x: sum(x) * 1.5
      
      # Predict the LTV using the model
      ltv = model(transaction_data[0])
      
      return ltv