To subtract 18.72 from 40, follow these steps:

Step 1: Write down the numbers
- Write down the number 40, which is the number from which we will be subtracting.
- Write down the number 18.72, which is the number that will be subtracted from 40.

Step 2: Set up the subtraction problem
- Write the larger number (40) on top and the smaller number (18.72) below it, lining up the decimal points.

40
- 18.72

Step 3: Subtract the whole numbers
- Start by subtracting the numbers in the ones place value. In this case, there are no whole numbers to subtract.

Step 4: Subtract the decimal numbers
- Begin by subtracting the numbers in the tenths place value. In this case, 2 minus 2 equals 0.
- Move on to the hundredths place value. In this case, 8 minus 7 equals 1.
- Finally, subtract the thousandths place value. In this case, 1 minus 8 is not possible, so we need to borrow from the tens place.

Step 5: Borrow from the tens place
- Since we cannot subtract 8 from 1, we need to borrow from the tens place.
- In the tens place, we have 4, so we can borrow 1 from it, making it 3.
- We then add 10 to the ones place, which gives us 11.

Step 6: Subtract the borrowed value
- Now, we can subtract 8 from 11, which gives us 3.

Step 7: Combine the results
- Combine the whole number result (0) with the decimal result (0.3) to get the final answer.

Therefore, subtracting 18.72 from 40 gives us the answer 21.28, which is option B.