Math puzzles regularly spark lively debates across social media platforms. Captions like “DON’T USE PEN AND PAPER: $90 – 10 \div 2 + 8$“ challenge viewers to test their mental math skills and recall standard arithmetic rules.
While the expression looks simple at first glance, people often arrive at different answers depending on whether they strictly follow the standard order of operations or simply calculate from left to right.
Step-by-Step Mathematical Solution
To solve $90 – 10 \div 2 + 8$ correctly, we apply the standard order of operations (PEMDAS / BODMAS):
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Parentheses / Brackets
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Exponents / Orders
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Multiplication and Division (from left to right)
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Addition and Subtraction (from left to right)
[ $90 - 10 \div 2 + 8$ ]
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[ Step 1: Divide $10 \div 2$ ]
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[ $90 - 5 + 8$ ]
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[ Step 2: Subtract & Add Left to Right ]
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[ $85 + 8 = 93$ ]
Step-by-Step Breakdown
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Step 1 (Division First): Identify the division operation $10 \div 2$.
$$10 \div 2 = 5$$Substituting back into the equation gives:
$$90 – 5 + 8$$ -
Step 2 (Subtraction and Addition Left to Right): Perform the remaining operations in order from left to right.
First, perform the subtraction:
$$90 – 5 = 85$$Then, perform the addition:
$$85 + 8 = 93$$ -
Final Answer: 93
Common Misconceptions and Errors
Why do these arithmetic posts go viral with conflicting answers?
| Common Mistake | Incorrect Calculation | Wrong Result |
| Calculating Strictly Left-to-Right | Performs $90 – 10 = 80$, then $80 \div 2 = 40$, then $40 + 8$ | 48 |
| Performing Addition Before Subtraction | Calculates $10 \div 2 = 5$, then adds $5 + 8 = 13$, then $90 – 13$ | 77 |
| Correct Method (PEMDAS / BODMAS) | Divides $10 \div 2 = 5$, then evaluates $90 – 5 + 8$ left to right | 93 |
Key Takeaways and Conclusion
When evaluating mathematical expressions, Multiplication and Division always take precedence over Addition and Subtraction, regardless of their position in the equation. Once only addition and subtraction remain, you simply evaluate from left to right to arrive at the correct answer: 93.
Frequently Asked Questions (FAQ)
What is the correct answer to $90 – 10 \div 2 + 8$?
The correct answer is 93.
Why do some people get 48?
People arrive at 48 by ignoring the order of operations and calculating straight from left to right: $(90 – 10) = 80 \rightarrow 80 \div 2 = 40 \rightarrow 40 + 8 = 48$.
Does Addition always come before Subtraction in PEMDAS?
No. In PEMDAS/BODMAS, Addition and Subtraction have equal priority and are performed strictly from left to right as they appear in the expression.
