Exercise 175 Mental Subtraction

As with mental addition, mental subtraction can be done from left to right too. Whereas the main problem in normal addition is the need to carry numbers, in subtraction it is borrowing.

With the following problems, there is no borrowing necessary. So practice subtracting these by proceeding from left to right:

96 74 97 49 86794 74698765

Now practice from left to right with the following, but when you need to borrow a number (like subtracting 9 from 5) change to 5 to 15 and subtract the 9. After writing the answer down (6), put a slash through the number preceding it. The slash through the number indicates that it is to be read as if it were actually one number less than what it says. For example:

957 8845 7523 8247 3487 7834 9347

Practice until it becomes natural to pair and slash, and soon you'll be able to see the entire process in your mind as well.

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