Below is an instruction that describes a task. Write a response that appropriately completes the request in English. Please answer in English.

### Instruction:
During a unit on probability, a teacher surveys her class asking her students two questions.  The first question was whether they prefer dogs or cats, and the second question was whether they movies or video games.  Of the 30 students in her class, 50% chose dogs and video games as their two answers, while 10% chose dogs and movies as their two answers.  In total, how many students in this teacher's class prefer dogs over cats?

### Response:
50% of the students chose dogs and video games, or 1 - 1 = <<50*.01*30=15>>15 students.
Similarly, 10% of the students chose dogs and movies, or 1 - 1 = <<10*.01*30=3>>3 students.
Because these are the only two ways that a student could indicate their preference for dogs, and because these groups do not overlap, a total of 15 + 3 = <<15+3=18>>18 students favor dogs over cats.
#### 18