This question tests percent increase calculations. The appropriate strategy is to find the increase and divide by the original price, then convert to percent. The increase is 92 - 80 = 12; 12/80 = 0.15. Thus, 0.15 × 100 = 15%. The increase is 15% of the original. A common incorrect choice is 13%, which might result from dividing by the new price (12/92 ≈ 0.13) instead of the original. Another error could be 12%, confusing the dollar increase with the percent.

This question tests approximation of decimals to fractions. The appropriate strategy is to convert each fraction to a decimal and compare to 0.18. 9/50 = 0.18 exactly; 2/11 ≈ 0.1818 is close but not exact. Other options like 1/18 ≈ 0.0556 are farther. The closest is 9/50. A common incorrect choice is 2/11, tempting because it's slightly higher but not as exact as 9/50. Another error could be 11/50 = 0.22, which is an overestimate.

This question tests calculating with fractions and percents in a multi-step problem. First, find the number of juniors: 30 × (2/5) = 12 juniors. Then find 25% of the juniors who are in math club: 12 × 0.25 = 3 students. Therefore, 3 juniors are in the math club. A common error is calculating 25% of all 30 students (giving 7.5) or finding 2/5 of 25% of 30 (giving 3 by coincidence but with wrong reasoning).

This question tests understanding of successive percent changes. When calculating successive percent changes, we must apply each change to the result of the previous change, not to the original value. Starting with an original price of 100, a 20% increase gives us 100 × 1.20 = 120. Then, a 20% decrease from 120 gives us 120 × 0.80 = 96. The final price of 96 is 96% of the original price of 100. A common error is to think that a 20% increase followed by a 20% decrease returns to the original price, but this ignores that the decrease is calculated on the larger intermediate value.

This question tests addition of fractions and percents. The appropriate strategy is to convert percent to fraction and add to the given fraction. 40% = 2/5 = 4/10; 3/10 + 4/10 = 7/10. Assuming percentages are of the total with no overlap, the combined is 7/10. The result is 7/10. A common incorrect choice is 2/5, which is Tuesday's share alone, forgetting to add Monday. Another error could be 3/4, perhaps from misadding or converting incorrectly.

This question tests comparison of decimals, fractions, and percents. The appropriate strategy is to convert all to decimals for comparison. 5/8 = 0.625; 61% = 0.61; 3/5 = 0.6; others are 0.62, 0.615. The greatest is 0.625. A common incorrect choice is 0.62, close but less than 0.625. Another tempting error is 0.615, perhaps from underestimating 5/8.

This question tests combining fractions with percents. The tank starts 2/5 full, and we need to find what fraction remains after draining 30% of the current water. First, we calculate 30% of 2/5: 0.30 × 2/5 = 6/50 = 3/25. This is the amount drained. The amount remaining is 2/5 - 3/25. To subtract these fractions, we need a common denominator: 2/5 = 10/25, so 10/25 - 3/25 = 7/25. A common error would be to calculate 30% of the full tank capacity rather than 30% of the water currently in the tank.

This question tests fractions and percents in the context of proportions. The appropriate strategy is to represent the initial fraction and apply the percentage drain to the current amount. The tank is 3/5 full; draining 20% of that means removing 1/5 of the current water, leaving 4/5 of the original amount. Thus, (4/5) × (3/5) = 12/25 of the tank is full. The result is 12/25. A common incorrect choice is 2/5, which might tempt if forgetting to apply the drain to the current amount and subtracting percentages directly. Another error could be 9/25, perhaps from miscalculating the remaining fraction as 3/5 minus 20% of the whole tank instead.

This question tests percents with successive changes. The appropriate strategy is to apply the increase and decrease multipliers sequentially. Increasing by 10% multiplies by 1.1; decreasing by 10% multiplies by 0.9. The overall factor is 1.1 × 0.9 = 0.99. The result is 99% of the original. A common incorrect choice is 100%, tempting because one might think the changes cancel, but the decrease is on a larger base. Another error could be 90%, from subtracting 10% twice incorrectly.

This question tests successive percent changes applied to a salary. Starting with an original salary of 100, a 10% decrease gives us 100 × 0.90 = 90. Then, a 10% increase from 90 gives us 90 × 1.10 = 99. The final salary of 99 is 99% of the original salary of 100. This demonstrates that successive percent changes of equal magnitude but opposite signs do not cancel out. The key insight is that the increase is calculated on the smaller intermediate value (90), so it doesn't fully restore the original amount. Many students incorrectly assume the changes cancel to give 100%.