In a group of 1500 people, 585 have blood type a, 165 have blood type b, 690 have blood type o, and 60 have blood type ab. what is the probability that a person chosen at random from this group has type b blood?

When calculating probability you need to remember that probability is:
something that you are observing divided by total number of participants.
The number you get is relative value(has value between 0 and 1) and if you want to get it in percentage (more clearly) just multiply it with 100.

In this case we are observing blood type B persons. Number of them is 165.
Total number of persons is 1500. Therefore our calculation looks like this:

165/1500 = 0.11    or  the probability is 0.11*100=11%