It would take 10 hours just like it did for Karl .
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%
Puedes multipilar 1/4 con 4, que te da 4/16. Sumas los dos y quedas con 11/16. Restas 16 de 11, y tu resultado sera 5/16.