Topic 9 of 15, EHL Maths & Excel final (AP)
Δ
Single % change (growth rate)
A single percentage change links the initial value Ko, the final value Kt and the rate r. If you know two of them, you can find the third.
When the exam asks for it
- “increase / decrease by …%”, “growth rate”, “what was the value before the change?”
Formulas
Final value
Kt = Ko*(1+r)Initial value
Ko = Kt/(1+r)Rate
r = (Kt-Ko)/KoWorked example
D2fx
=200*(1+15%)| A | B | C | D | E | |
|---|---|---|---|---|---|
| 1 | Case | Ko | r | Kt | |
| 2 | Find Kt | 200 | +15% | 200*(1+15%) = 230 | |
| 3 | Find Ko | ? | +15% | 230/(1+15%) = 200 | |
| 4 | Find r | 80 | ? | (92-80)/80 = 15% (Kt = 92) | |
| 5 | Decrease | 200 | -10% | 200*(1-10%) = 180 | |
| 6 |
Sheet1
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Step by step
=200*(1+15%)230=230/(1+15%)200=(92-80)/8015%
⚠ Exam traps
- A decrease is a negative r: 1+r becomes 1-10% = 0.9.
- To find the value before a change, divide by (1+r). Taking 15% off 230 gives 195.5, which is wrong.
Practice question
Revenue is 345,000 CHF after a 15% increase. What was it before?
Show answer
✓ 345,000/(1+15%) = 300,000 CHF.
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