Correct Answer: B

Rationale: To convert kilometers to meters, you need to multiply the number of kilometers by 1000 since there are 1000 meters in 1 kilometer. Therefore, 5 kilometers is equal to 5 1000 = 5000 meters. Choice A (1000) is incorrect because it represents the number of meters in 1 kilometer, not 5 kilometers. Choice C (10000) is incorrect as it is the result of multiplying 10 (not 5) by 1000. Choice D (500) is incorrect as it represents half the correct conversion value.

Correct Answer: B

Rationale: In the metric system, there are 1000 grams in 1 kilogram. To convert kilograms to grams, you multiply the number of kilograms by 1000. Therefore, 4 kilograms would equal 4000 grams. Choice A (400 grams) is incorrect because it is the result of multiplying by 100, not 1000. Choice C (40 grams) is incorrect as it is the result of dividing by 1000 instead of multiplying. Choice D (100 grams) is incorrect as it is a random figure and not the correct conversion from kilograms to grams.

Correct Answer: A

Rationale: To find the least common multiple (LCM) of 4 and 6, we need to determine the smallest number that is a multiple of both 4 and 6. The multiples of 4 are: 4, 8, 12, 16, 20, 24, ... The multiples of 6 are: 6, 12, 18, 24, ... The least common multiple is the smallest number that appears in both lists. In this case, the least common multiple of 4 and 6 is 12, not 24. Therefore, the correct answer is 24. Choice B (12) is actually the least common multiple of 4 and 3, not 4 and 6. Choices C (6) and D (3) are not multiples of both 4 and 6, so they are incorrect.

Correct Answer: C

Rationale: Reducing the rat's speed by 50% means it will take twice as long to finish the maze. Since the original time is 3 minutes, doubling that gives 6 minutes. Therefore, the total time will be 6 minutes, making the correct answer C. Choice A (3 minutes) is the original time it takes the rat to finish the maze, not the time with the backpack. Choice B (4 minutes) is not correct as reducing the speed by 50% would double the original time. Choice D (1.5 minutes) is incorrect as halving the time is not the effect of reducing the speed by 50%.

Correct Answer: C

Rationale: To find three tenths of 90, you need to multiply 90 by 0.3 (since three tenths equals 0.3). Therefore, 90 * 0.3 = 27. This calculation shows that the correct answer is 27. Choice A (18), choice B (45), and choice D (36) are incorrect as they do not represent three tenths of 90.