In your head, you should be able to add 15 and 5. The answer is 20. You should also be able to work out 20 + 9. The answer is 29. The trick to doing well at these types of questions is finding the quickest and easiest method of working them out. It will come with practice :)
She has 3 problems. Mate (I think you mean Maths), Social Studies and Science. Unless you refer specifically to the number of problems in each of the three major problems, which would be 29. If this was a logic game question then it would be that she had no problems in homework because it's not specified that the problems in the three subjects were homework.
