It's called mental math.
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The opposite of add is subtract.
You add or subtract only the numerators
It depends on if you want to add them, subtract, multiply, divide, or something else with them.
It depends on how the rest of the question is worded. If it says "how many were left," for example, that means to subtract. On the other hand, if it says something like "how many were added," it means to add.
By the signs: 2+2(Add) 2-2(Subtract) +=add -=subtract