Math Problems and Brain Teasers

If A solves 24 problems in 15 minutes, her rate is 24/15 = 1.6 problems per minute. In 40 minutes, she can solve 1.6 × 40 = 64 problems.

5% off Rs 60 = Rs 57. Adding 5% sales tax: 57 × 1.05 = Rs 59.85 = Rs 59 and 85 paise.

If population doubles every 10 years from 1960 to 1990, that's 30 years = 3 periods. Starting from P, after 3 periods: P × 2^3 = 8P. Percent increase = ((8P - P)/P) × 100 = 700%.

Using the compound interest formula for population: P = 15625 * (1 + 0.04)^3. This calculates to 15625 * (1.04)^3 = 15625 * 1.124864 = 17576. The other options are calculation errors or near-misses.

To convert a ratio or fraction to a percentage, multiply by 100. (5 / 4) * 100 = 1.25 * 100 = 125%. 80% is the inverse ratio (4:5), and 12.5% is a decimal placement error.

The lights will all show red again at the LCM of 50, 35, and 30. Prime factorization: 50 = 2 × 5², 35 = 5 × 7, 30 = 2 × 3 × 5. LCM = 2 × 3 × 5² × 7 = 1050. This means all three lights will simultaneously be red again after 1050 seconds (17 minutes 30 seconds). Option D is correct.

Let P be the principal and r be the first rate. Amount A = P(1 + rt/100). So, P(1 + 4r/100) = 1088 and P(1 + 3(r+3)/100) = 1088. Solving these equations yields r = 9%. The other options do not satisfy the equal amount condition.

This is a wordplay riddle. If you physically take away three apples from a group, you are now holding those three apples. The question asks how many 'you' have, not how many are left in the original pile.

This is a trick question based on biology. Roosters are male chickens and do not lay eggs; only hens lay eggs. Therefore, no egg exists to fall inside or outside the well. 'None of the above' is the only logical choice.

The digit 1 appears 20 times between 1 and 100: 1, 10, 11 (twice), 12-19 (eight times), and 21, 31, 41, 51, 61, 71, 81, 91 (nine more times). Since 20 isn't listed, 'None of the above' is correct.

This is a classic riddle. While the previous examples use color to describe the building material, a 'greenhouse' is a specific structure used for growing plants, which is primarily made of glass to trap heat. 'Green bricks' is a distractor based on the linguistic pattern.

This is a trick question about the 12-hour clock cycle. All listed times occur after 3:00 p.m. at some point: 9:00 p.m. same day, 12:00 midnight that night, 9:00 a.m. next morning, and 3:00 p.m. next day.

Every month has at least 28 days. February typically has exactly 28 days, while all other months have 30 or 31 days. Therefore, all listed months have 28 days.

If the Vice President dies, the President remains in office. The VP position would be vacant until filled, but the President is unchanged. The Speaker only becomes President if both President and VP are unable to serve.

The boy has 11 fingers, and each finger has one fingernail. Thumbs are counted separately from fingers and also have nails, but the question specifically asks for fingernails, not thumbnails.

This is a classic trick question. The pilot's name is 'You' - the question explicitly states 'You're the pilot' at the beginning. No calculation is needed.

The actual time at home was 7:05. Mirror reflection shows 4:55 (mirrored horizontally). Joey rides for 20 minutes and arrives at camp at 7:25, which is exactly 2.5 hours after 4:55. Working backwards from the given constraints confirms 7:25 as the arrival time.

To average 60 km/h over the entire journey, you'd need to complete the whole trip in a specific time. If you've already taken time to cover half the distance at 30 km/h, it's mathematically impossible to reach an average of 60 km/h for the full distance - you would need infinite speed for the remaining half.