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S1Q1 · Compute Electricity Bill

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Function: compute_electricity_bill(units: int) -> float

Core idea: three-slab billing - each slab has a per-unit rate and an optional fixed charge.

def compute_electricity_bill(units: int) -> float:
    if units <= 200:
        cost = units * 0.5
    elif units <= 400:
        cost = units * 0.75 + 150
    else:
        cost = units * 0.90 + 300
    return cost

Key rules: - ≤ 200 units → 0.5 per unit, no fixed charge - ≤ 400 units → 0.75 per unit + 150 fixed - > 400 units → 0.90 per unit + 300 fixed

Problem Statement

Problem

Write a function compute_electricity_bill(units) that returns the total electricity bill based on the slab rates.

Examples:

Input 
200
Output 
100.0
Input 
412
Output 
670.8

Problem statement error

The problem states compute_electricity_bill(210) -> 236.50. This is incorrect - no formula consistent with the given rates produces 236.50 for 210 units. The flat-rate formula gives 210 × 0.75 + 150 = 307.50. The correct answer for 210 units is 307.50.

Tracing all examples

units Slab Formula Result
200 ≤ 200 200 × 0.5 100.0 ✓
210 ≤ 400 210 × 0.75 + 150 307.5
412 > 400 412 × 0.90 + 300 670.8 ✓

Solution approaches

def compute_electricity_bill(units: int) -> float:
    if units <= 200:
        cost = units * 0.5
    elif units <= 400:
        cost = units * 0.75 + 150
    else:
        cost = units * 0.90 + 300
    return cost

def compute_electricity_bill(units: int) -> float:
    if units <= 200:
        rate         = 0.5
        fixed_charge = 0
    elif units <= 400:
        rate         = 0.75
        fixed_charge = 150
    else:
        rate         = 0.90
        fixed_charge = 300
    return units * rate + fixed_charge

compute_electricity_bill = lambda units: (
    units * 0.5        if units <= 200 else
    units * 0.75 + 150 if units <= 400 else
    units * 0.90 + 300
)

Key takeaways

01

if-elif chain - check lower slabs first

Since elif units <= 400 only runs when units > 200 (the first condition already failed), the slabs don't overlap. Always order conditions from lowest to highest slab boundary.

02

Flat rate vs tiered billing

This problem uses a flat rate - the rate applies to all units, not just the units above the previous threshold. `210 × 0.75 + 150` charges 0.75 per unit for all 210 units. Tiered billing (like income tax) would only charge 0.75 on the extra 10 units.

03

Return float - don't truncate

The template's return cost suggests the result should be a float. Python arithmetic with 0.5, 0.75, 0.90 already produces floats - no explicit conversion needed.