What you need to know
Depreciation is the loss in value of an asset over time. With straight-line (flat-rate) depreciation the same amount D is lost each period: V = V₀ − Dn. With declining-balance (reducing-balance) depreciation the same percentage r is lost each period: V = V₀(1 − r)n, so the value falls quickly at first and never quite reaches zero.
An annuity is a series of equal payments M made at regular intervals with compound interest at rate r per period. Its future value is what the payments grow to (a savings plan); its present value is the single amount today that is worth the same as all the payments (a loan). A reducing-balance loan is an annuity in reverse: each period, interest is added to the balance, then the repayment is subtracted. The same step written as a recurrence relation is Vₙ₊₁ = Vₙ(1 + r) − M.
| Model | Formula | Notes |
|---|
| Straight-line depreciation | V = V₀ − Dn | D = amount lost per period |
| Declining-balance depreciation | V = V₀(1 − r)n | r = rate per period as a decimal |
| Future value of an annuity | FV = M × (1 + r)n − 1r | payments at the end of each period |
| Present value of an annuity | PV = M × 1 − (1 + r)−nr | also the amount that can be borrowed |
| Loan repayment | M = PV × r1 − (1 + r)−n | rearranged from the PV formula |
| Recurrence (loan) | Vₙ₊₁ = Vₙ(1 + r) − M, V₀ = amount borrowed | interest first, then repayment |
A $30,000 car depreciates by $4500 per year (straight line). Find its value after 4 years, and when it is written off.
- V = 30,000 − 4500 × 4 = 30,000 − 18,000 = $12,000
- Written off when V = 0: 30,000 ÷ 4500 = 6.67, so during the 7th year (after 6 years it is still worth $3000).
The same $30,000 car depreciates at 20% per year (declining balance). Find its value after 4 years.
- V = 30,000 × (1 − 0.2)4 = 30,000 × 0.84 = 30,000 × 0.4096 = $12,288
- Compare: straight-line gave $12,000 after 4 years, but declining balance loses more early on ($6000 in the first year versus $4500) and less later.
Savings plan: $500 is deposited at the end of every month for 10 years at 6% per year compounded monthly. Find the future value.
- r = 0.06 ÷ 12 = 0.005; n = 10 × 12 = 120
- FV = 500 × 1.005120 − 10.005 = 500 × 1.81939… − 10.005 = 500 × 163.879…
- FV = $81,939.67. The deposits total $60,000, so $21,939.67 is interest.
Home loan: $400,000 is borrowed at 6% per year compounded monthly over 30 years. Find the monthly repayment and the total interest.
- r = 0.005; n = 360
- M = 400,000 × 0.0051 − 1.005−360 = 20001 − 0.16604… = 20000.83396… = $2398.20 per month
- Total repaid = 2398.20 × 360 = $863,352
- Total interest = 863,352 − 400,000 = $463,352 — more than the amount borrowed.
Recurrence relation: $20,000 is borrowed at 12% per year compounded monthly, with repayments of $600 per month. Find the balance after 3 months.
- r = 0.12 ÷ 12 = 0.01, so Vₙ₊₁ = 1.01Vₙ − 600 with V₀ = 20,000.
- V₁ = 1.01 × 20,000 − 600 = 20,200 − 600 = 19,600
- V₂ = 1.01 × 19,600 − 600 = 19,796 − 600 = 19,196
- V₃ = 1.01 × 19,196 − 600 = 19,387.96 − 600 = $18,787.96
- Interest in month 1 was $200 and the principal fell by $400; each month slightly more of the $600 goes to principal.
Present value: how much must be invested now at 5% per year compounded annually to provide $2000 at the end of each year for 5 years?
- PV = 2000 × 1 − 1.05−50.05 = 2000 × 1 − 0.78353…0.05 = 2000 × 4.3295…
- PV = $8658.95. The payments total $10,000; the rest comes from interest earned on the money still invested.
Common mistake: using the annual rate with monthly periods. A 6% per year loan repaid monthly uses r = 0.005 and n in months. Also keep (1 + r)n unrounded until the end — rounding 1.005360 to 6.02 changes a repayment by cents and the total interest by dollars. Another common mistake: forgetting that total interest = (repayment × number of repayments) − amount borrowed. And with declining-balance depreciation, the value never reaches zero, so a question asking "when is it worth less than $5000?" needs logarithms or a table, not a division.
Check yourself
- A $12,000 machine depreciates by $1500 per year. Find its value after 5 years.
- A $12,000 machine depreciates at 15% per year (declining balance). Find its value after 3 years.
- V₀ = 5000 and Vₙ₊₁ = 1.02Vₙ − 300. Find V₁ and V₂.
- Find the future value of $1000 deposited at the end of each year for 3 years at 4% per year
- $10,000 is borrowed at 12% per year compounded monthly over 12 months. Find the monthly repayment.
- Find the total interest paid on the loan in question 5.
Answers: 1. 12,000 − 7500 = $4500 2. 12,000 × 0.853 = $7369.50 3. V₁ = 5100 − 300 = 4800; V₂ = 4896 − 300 = 4596 4. 1000 × 1.043 − 10.04 = $3121.60 5. M = 10,000 × 0.011 − 1.01−12 = $888.49 6. 888.49 × 12 − 10,000 = $661.88