One of the basic trouble spots in subtracting decimals is determining the value of each number and ultimately the largest number, if the numbers do not have any digits to the left of the decimal. The first step to accomplish this is to look at the number of zeros after the decimal in each number. The number with the least zeros directly to the right of the decimal is the largest. If the two numbers have the same number of zeros, then the first digit other than zero is used to determine the largest of the two numbers. The number with the bigger digit is the largest number.
Add zeros to the right of the number with the least number of digits to the right of the decimal so that the two numbers that are being subtracted have the same number of digits. For instance, if one number is 11.245 and the other number is 11.5, add two zeros to the end of 11.5, to make 11.500. Don't worry; adding zeros to the number in this way does not change the value.
As basic as it sounds, one of the basic keys to understanding how to subtract decimals is to sit down with a piece of paper and a pencil. Write the largest number on top of the smaller number and draw a line underneath. When writing the two numbers, match the decimals so they are in the same place vertically in the equation. Before subtracting, place a decimal in the answer, exactly vertically below the decimal in the equations. This will help you keep track of the decimal.
Add the answer that you have found to the smaller of the two decimal numbers. The answer should be the larger of the two numbers that you subtracted. If not, you made a mistake. Check your math.