Adding and subtracting decimals is the same as adding and subtracting whole numbers, except that you need to make sure all numbers have the same number of digits before starting and that you don’t move the decimal point.
For example if you were asked
7.4 + 9 + 3.768 = ?
It will be easiest to add zeros so that all numbers have the same number of digits as the number with the most digits, in this case 3.768 has 4 digits.
You should set it up the same way for subtraction since it will make borrowing easier to visualize.
7 – 5.05 = ?