"Arithmetic Tutor - Subtraction" is an Android app that helps grade school students to learn and master the subtraction of whole numbers. The student selects the options for the exercise, such as the number of digits for each operands, whether to enable borrow, and number of problems in each exercise. The app then generates the operands for each problems in the exercise based on the options selected. The student is given the opportunity to solve it, and the app will check the answer. If the answer is incorrect, the student can either try again, or ask the app to show where the mistake(s) is/are, or ask the app to show the steps to solve the problem. All instructions are verbal, with the option of displaying the verbal instructions as text. The score for the exercises are recorded, and can be retrieved to find out the student's progress.

The method used in this app employs the technique sometimes called compensating, which is easier to learn than the traditional method of regrouping. The idea is that adding and subtracting the same number will not change the result of the subtraction. But even more important is not doing the subtraction of the borrow immediately, which avoids the cascading effect of borrowing from higher places, as in the problem of (2000-1234). This is similar to addition where the carry is not added to the next place immediately until you are adding the digits for the next place.

In general, the functionalities of this app is the same as "Arithmetic Tutor - Addition", and will not be repeated here. The following are the screen shots and descriptions of the most important functionalities that are specific to this app.

The Main UI page lets the student to select the operands size, whether to enable borrow, the number of problems in the exercise, and whether to display the verbal instructions as text. When the student is satisfied with all the options, he/she can start the exercise by touching the Start Exercise button.

The exercise starts with generating a problem with operands that is generated randomly, but at the same time satisfies the criteria selected in the options from the main UI page. If borrow is enabled, the operands are generated in such a way that not all place position subtraction will result in a borrow from the next place all the time. This is to prevent the student from mistakenly believe that borrow is always needed. The student can solve the problem by dragging and dropping digits from the bottom row of digits, onto the borrow and answer digit positions indicated by a faint grid of boxes displayed during the drag and drop process. If a digit was landed on an incorrect position by mistake, it can be removed by dragging it away from any grid position. Once the student is satisfied with the answer, a long press on a display area that does not have a digit that can be dragged will invoke the context menu.

When "What's Wrong" is finished, the display will be shown as in the left, where any incorrect answer or borrow digit(s) will be highlighted in red with a red cross on the digit position.

Note that once the app has shown the student where the mistake(s) is/are, in the answer, the student will no long be allowed to change the answer by dragging or dropping digits. The student can, however, still invoke the context menu. For example, to select Check Answer again to see where the mistake(s) is/are made, or choose another context menu option.

Also, while the app is showing where the mistake(s) is/are, the student can invoke the context menu and select the "Stop What's Wrong" item to terminate What's Wrong.

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