## Explanation

The core of this formula is a single IF statement that checks the amount in column B against the base limit:

```
=IF(B5<limit
```

If TRUE, the formula simply multiplies the amount in B5 by tax rate:

```
B5*rate
```

If FALSE, the formula applies the tax rate to the amount over 1000, then adds the fixed amount:

```
(B5-limit)*rate+fixed)
```