If two students, Sarah and Tom, swap their positions and Sarah is now 10th from the top and Tom is 20th from the bottom, what was Tom's original position from the top?
Answer: C. 19th
If Sarah is now 10th from the top, the total number of students is 10 + 20 - 1 = 29. Tom's original position from the top is 29 - 20 + 1 = 10.