If you want to use a number line to add and subtract, it can be done with a slide rule. But it is much easier to use an electronic calculator.
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You add or subtract (or use other operators) depending on what is required.
To subtract a positive number, you go the specified number of units to the left. To subtract a negative number (which is the same as adding the corresponding positive number), you go to the right.
First, subtract the absolute values of the integers, then use the greater absolute value's sign.
When you want to add or subtract unlike fractions.
(9*9)+9+9+9/9 = 100