Solve with due regards to significant figures
`4.0 xx (10^-4) - 2.5 xx 10^(-6)`.

Explanation

To solve the expression \( 4.0 \times 10^{-4} - 2.5 \times 10^{-6} \) with due regard to significant figures, we will follow these steps:

### Step 1: Align the numbers to the same power of ten
To perform the subtraction, we need to express both numbers with the same exponent. The larger exponent is \( -4 \), so we will convert \( 2.5 \times 10^{-6} \) to the same power of ten:

\[ 2.5 \times 10^{-6} = 0.0025 \times 10^{-4} \]

### Step 2: Rewrite the expression
Now we can rewrite the expression:

\[ 4.0 \times 10^{-4} - 0.0025 \times 10^{-4} \]

### Step 3: Perform the subtraction
Now we can subtract the two numbers:

\[ (4.0 - 0.0025) \times 10^{-4} = 3.9975 \times 10^{-4} \]

### Step 4: Determine significant figures
Next, we need to consider significant figures. The number \( 4.0 \) has 2 significant figures, and \( 2.5 \) also has 2 significant figures. The result should be reported with the least number of decimal places from the original numbers.

### Step 5: Round the result
The result \( 3.9975 \) should be rounded to 2 significant figures:

\[ 3.9975 \approx 4.0 \times 10^{-4} \]

### Final Answer
Thus, the final answer, considering significant figures, is:

\[ \boxed{4.0 \times 10^{-4}} \]

---