# Writing small, focused functions
def calculate_subtotal(item_prices):
    return sum(item_prices)

def calculate_tax(subtotal, tax_rate):
    return subtotal * tax_rate

def calculate_total_cost(item_prices, tax_rate):
    subtotal = calculate_subtotal(item_prices)
    tax = calculate_tax(subtotal, tax_rate)
    total_cost = subtotal + tax
    return total_cost