📊 Mean (average)
Mean = sum of the numbers ÷ how many numbers.
The mean of 6, 8, 10, and x is 9.
Four numbers, so the sum is 9 × 4 = 36.
6 + 8 + 10 + x = 36 → x = 12.
🏷️ Percents and tax
Do discounts first, then tax, unless the problem says otherwise.
A $40 shirt, 25% off, then 8% tax on the sale price.
25% of 40 is $10, so the sale price is $30.
8% of 30 is $2.40. Total = $32.40.
🧮 Quick checks
- Of means multiply: 20% of 50 = 0.20 × 50 = 10.
- A fraction of a number: (1/2) × 18 = 9.
- If your answer is way bigger than the starting number after a “discount,” you added instead of subtracted.