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?
How many students in this teacher's class prefer dogs over cats? ** The 50% of the class that chose dogs and video games amounts to 30*0.5 = <<50*.01*30=15>>15 students who prefer dogs over cats.
How many students in this teacher's class prefer dogs over cats? ** Similarly, the 10% that chose dogs and movies amount to 30*0.1 = <<30*0.1=3>>3 students who prefer dogs over cats.
How many students in this teacher's class prefer dogs over cats? ** 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 in this teacher's class.
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