To solve this problem, we can find the individual typing rates for Susan and Mary, and then add them together to find their combined typing rate.
Susan can type 10 pages in 5 minutes, which means she types at a rate of 10/5 = 2 pages per minute.
Mary can type 5 pages in 10 minutes, which means she types at a rate of 5/10 = 0.5 pages per minute.
To find their combined typing rate, we add their individual rates:
Combined typing rate = Susan's typing rate + Mary's typing rate
Combined typing rate = 2 + 0.5
Combined typing rate = 2.5 pages per minute
Now, we can use the combined typing rate to find the number of pages they can type in 30 minutes:
Number of pages = Combined typing rate * Time
Number of pages = 2.5 * 30
Number of pages = 75
Therefore, Susan and Mary can type 75 pages in 30 minutes.
The correct answer is B) 75.