Evaluate:
`35.9 - 2.1 xx 0.9 + 17.2 div 0.4 - 0.4`

Answer

To evaluate the expression `35.9 - 2.1 x 0.9 + 17.2 ÷ 0.4 - 0.4`, we will follow the order of operations, often remembered by the acronym BODMAS (Brackets, Orders, Division and Multiplication, Addition and Subtraction).

### Step-by-Step Solution:

1. **Identify the operations**: The expression contains subtraction, multiplication, addition, and division. According to BODMAS, we will perform division and multiplication before addition and subtraction.

2. **Perform the division**:
- Calculate `17.2 ÷ 0.4`.
- To do this, we can convert it to a fraction:
\[ 17.2 ÷ 0.4 = \frac{17.2}{0.4} = \frac{172}{4} = 43 \]

3. **Rewrite the expression**:
- Now substitute `43` back into the expression:
\[ 35.9 - 2.1 x 0.9 + 43 - 0.4 \]

4. **Perform the multiplication**:
- Calculate `2.1 x 0.9`.
- Multiply as follows:
\[ 2.1 x 0.9 = 21 x 9 = 189 \]
Since both numbers have one decimal place, we place the decimal two places from the right:
\[ 2.1 x 0.9 = 1.89 \]

5. **Rewrite the expression again**:
- Substitute `1.89` into the expression:
\[ 35.9 - 1.89 + 43 - 0.4 \]

6. **Perform the subtraction and addition from left to right**:
- First, calculate `35.9 - 1.89`:
\[ 35.9 - 1.89 = 34.01 \]
- Next, add `34.01 + 43`:
\[ 34.01 + 43 = 77.01 \]
- Finally, subtract `0.4` from `77.01`:
\[ 77.01 - 0.4 = 76.61 \]

7. **Final Result**:
- The final answer is:
\[ \boxed{76.61} \]