Quick instructions       Clicking on the buttons "C1", "C2" and "C3" we can change the colors of the squares that represent hundreds, tens and units in the added numbers and their result. Actually, the added numbers need colors only for tens and units. All selected colors are shown near the upper right corner of the applet. To achieve the best visualization it is a good idea to select different colors for hundreds, tens and units.      To add 16 and 25 select 1 from the left digit column, click on it and click on the first "10" button in the second row of the applet. Then the first square in the column under the "10" button will be colored showing that 16 contains a one full ten. Do the same with 6 but this time click on the first "1" button in the second row of the applet. Now the first six squares right below the "1" button will be colored. These two operations will also produce the number 16 in the last row of the applet.       Do the same with the number 25 using the second "10" and "1" buttons in the second row of the applet. Proper numbers of squares will be colored representing the number 25 and now in the last applet row there will be visible the expression 16 + 25.       At this stage you can modify any of the inserted numbers. As a matter of fact an order of digit insertion and modification is irrelevant. You can do it until you click on one of the two equality buttons. This click triggers the appearance of the addition result and the colored squares visualizing it. Simultaneously the applet is ready for the next addition operation.       Besides of addition this applet can perform the subtraction of two digit nonnegative numbers. To enter the subtraction mode click on the "+" button in the upper left corner of the applet. All three "+" buttons will turn into "-" buttons and the applet will be able to perform the subtraction. If the result is negative, the negative sign will appear in the front of it and the colored squares (not white) will appear as the background.       Clicking on the "-" button in the upper left corner of the applet restores the addition mode.