If two students, Mark and Lucy, interchange their ranks in a group of 40 students where Mark is 10th from the top and Lucy is 31st from the bottom, what will be Mark's new rank from the bottom after the interchange?
Answer: A. 30th
After the interchange, Mark's new rank from the bottom will be 40 - 10 + 1 = 31, but since he takes Lucy's position, he will be 30th from the bottom.