Hacking question: Only people with extremely high IQ can solve this puzzle

Only smart people with extremely high IQ can answer this question correctly, are you one of them?
The question below is a simple calculation in elementary school textbooks, but not all adults can find the correct answer in a short time. Even many people have to shake their heads in frustration because they cannot find a number that satisfies the calculation conditions given in the problem.

Only extremely high IQ people can solve this puzzle – 1
The problem gives 3 cups of water (green, yellow, black) with the given data of 3 blue cups added together = 42; green cup + 2 yellow cups = 24; yellow cup + black cup + blue cup = 26. The question is how much is a blue cup + black cup – yellow cup?

Please observe carefully to give the correct answer. My advice is to take a pen and paper and calculate carefully to avoid confusion.

If you find the answer, quickly enroll yourself in the top smartest people by leaving your answer in the comment box below.