Household arithmetic (घरायसी अङ्कगणित) questions give you the tariff or exchange rate in the question; your job is to apply it step by step. The rates below are examples for practice; always use the rates given in the exam question.
1. Electricity bill
- Units used = current meter reading − previous meter reading.
- Charges are in slabs: each slab is charged at its own rate.
- Add the fixed/service (minimum) charge.
| Example tariff (practice only) | Rate |
|---|---|
| First 20 units | Rs 4 per unit |
| Next 30 units (21–50) | Rs 7 per unit |
| Above 50 units | Rs 9 per unit |
| Service charge | Rs 50 per month |
Example: previous reading 4,325, current 4,400 → 75 units. Energy charge = 20 × 4 + 30 × 7 + 25 × 9 = 80 + 210 + 225 = 515. Bill = 515 + 50 = Rs 565.
2. Water bill
Example tariff: minimum Rs 100 for up to 10,000 litres; Rs 32 for every extra 1,000 litres. A house uses 16,000 litres: extra 6,000 litres → 6 × 32 = 192. Bill = 100 + 192 = Rs 292.
3. Telephone bill
Example: monthly rental Rs 200 and 300 minutes of calls at Rs 0.50 per minute = Rs 150. Subtotal Rs 350; add 13% VAT (Rs 45.50) → Rs 395.50.
4. Money exchange (मुद्रा विनिमय)
- Nepal Rastra Bank publishes exchange rates every day.
- Buying rate: the rate at which the bank buys foreign currency from you (lower).
- Selling rate: the rate at which the bank sells foreign currency to you (higher).
- Indian rupee is pegged: 1 INR = 1.60 NPR.
- If 1 USD = Rs 135, then 200 USD = 200 × 135 = Rs 27,000.
- Rs 40,500 to USD at 135 = 40,500 ÷ 135 = 300 USD.
- 5,000 INR = 5,000 × 1.6 = 8,000 NPR; 16,000 NPR = 16,000 ÷ 1.6 = 10,000 INR.
- Cross rate: 1 USD = Rs 135 and 1 EUR = Rs 150. 90 EUR = 13,500 NPR = 13,500 ÷ 135 = 100 USD.
5. More worked examples
All rates below are for practice only. In the exam, always use the rates given in the question.
| Question | Working | Answer |
|---|---|---|
| Electricity: 120 units with the tariff in section 1 | 20 × 4 = 80; 30 × 7 = 210; 70 × 9 = 630; total 920; add service charge 50 | Rs 970 |
| Late payment: the Rs 970 bill paid late with a 5% penalty | 970 × 5% = 48.50; 970 + 48.50 | Rs 1,018.50 |
| Water: 25,000 litres with the tariff in section 2 | Extra = 15,000 litres → 15 × 32 = 480; add minimum 100 | Rs 580 |
| Telephone: rental Rs 250 and 400 minutes at Rs 0.50, plus 13% VAT | 250 + 200 = 450; VAT = 450 × 13% = 58.50; 450 + 58.50 | Rs 508.50 |
| Exchange loss: sell 500 USD to the bank at Rs 134, later buy 500 USD back at Rs 135 | Received 500 × 134 = 67,000; paid 500 × 135 = 67,500 | Loss of Rs 500 |
The last example shows why the bank’s selling rate is higher than its buying rate: the difference (spread) is the bank’s income.
6. Step-by-step method for any bill question
- Step 1: find the units used (current reading − previous reading) or the total usage given.
- Step 2: split the units into slabs and charge each slab at its own rate. Do not charge all units at the highest rate.
- Step 3: add fixed charges such as minimum charge, service charge or rental.
- Step 4: add VAT, discount or penalty only if the question gives it, and on the right amount.
- Step 5: check the units: rupees and paisa, litres or units, and round only at the end.
7. Common mistakes
- Using the selling rate when the bank is buying from you (or the other way round).
- Multiplying when you should divide in currency conversion. Changing rupees into dollars means dividing by the rate.
- Forgetting the minimum or service charge.
- Adding VAT before adding all the other charges.
8. How this topic is asked in the exam
- Objective: calculate one bill or one currency conversion, often with four close answer options.
- Tricky: slab tariffs, penalties for late payment, and buying vs selling rates.
- Tip: write each slab on a separate line in rough work, as in the examples above, to avoid mistakes.
📌 Key points to remember
- Units = current reading − previous reading.
- Apply each slab at its own rate, then add service charge.
- Add VAT (13%) when the question says so.
- Bank buying rate is lower; selling rate is higher.
- 1 INR = 1.60 NPR; foreign → NPR multiply; NPR → foreign divide.
Test yourself: Household Arithmetic
Practise Electricity bill, Telephone bill and Money exchange (मुद्रा विनिमय) with 15 questions.