How many units of currency A did the outlet buy on that day?
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Correct answer: 1200
Let the base rates (units of local currency L per unit) be , , , matching the ratio . The outlet buys at 5% below base (base) and sells at 10% above base (base).
Pin down :
- Selling A brought in 88000 L at a selling rate of , so units of A sold .
- A's stock rose from 2500 to 3300, a gain of 800, so (A bought) (A sold) , giving A bought . The L spent buying A is .
- Sales of A and B are in ratio , so sale of B L. At B's selling rate , units of B sold .
- B's stock is unchanged, so B bought B sold , and L spent buying B .
- L used to buy A and B are in ratio : .
So the base rates are A = 200, B = 240, C = 2; buying rates 190, 228, 1.90; selling rates 220, 264, 2.20.
| Currency | Base | Buying | Selling | Units bought | Units sold |
|---|---|---|---|---|---|
| A | 200 | 190 | 220 | 1200 | 400 |
| B | 240 | 228 | 264 | 600 | 600 |
| C | 2 | 1.90 | 2.20 | 22000 | 19000 |
This question — units of A bought. With , A sold ; since A bought A sold , A bought .
💡 Teacher tip: Carry the whole rate scale as one unknown ; the two buy/sell ratios pin it down without ever needing the absolute base rates first.
Answer: 1200 units