(0.85% of 405) + (2.25% of 550) = $x$

### Concept & Formula The problem requires calculating percentages of decimal numbers and summing the results. The key insight is to convert the percentages into exact decimals or fractions to avoid calculation errors. $$x\% = \frac{x}{100}$$ ### Step-by-Step Solution **Given:** * First term: $0.85\%$ of $405$ * Second term: $2.25\%$ of $550$ **Calculation / Deduction:** * Convert the first percentage expression into standard multiplication: $\frac{0.85}{100} \times 405$ * Simplify it to decimals: $0.0085 \times 405 = 3.4425$ * Convert the second percentage expression: $\frac{2.25}{100} \times 550$ * Simplify it to decimals: $0.0225 \times 550 = 12.375$ * Add the two computed values together: $3.4425 + 12.375 = 15.8175$ * Compare $15.8175$ with the given options. Since it does not match any of the numeric options (the closest is $15.7150$), the correct choice is "None of these". ### Exam Strategy & Shortcut Instead of full decimal multiplication, you can use digital root or unit digit approximation if options are spaced out. However, since the options here are very tight and decimal-heavy, standard multiplication is the safest route. To multiply faster, treat $0.0085 \times 405$ as $85 \times 405$, then shift the decimal point four places to the left. $85 \times 400 = 34000$, and $85 \times 5 = 425$, yielding $34425$. Shifting decimals gives $3.4425$. ### Common Pitfall The single most common mistake is misplacing the decimal point when converting $0.85\%$ to a decimal (e.g., writing $0.085$ instead of $0.0085$). Always remember that the percentage sign itself represents division by $100$. ### Final Answer Therefore, the correct answer is **None of these**.