# Excel extras: data challenge

Import products, customers and January sales. Left-join on product and customer IDs. Calculate revenue = quantity × unit price, cost = quantity × unit cost, and margin = revenue − cost. Append February and refresh.

Only then import the P99 challenge: retain the row and flag its missing price/cost rather than silently treating it as zero.

Controls before the challenge:

```json
{
  "gennaio": {
    "righe": 5,
    "ricavi": 350,
    "costi": 178,
    "margine": 172
  },
  "gennaio_febbraio": {
    "righe": 10,
    "ricavi": 645,
    "costi": 326,
    "margine": 319
  }
}
```