Truly bizarre.
So how many people are going to answer 38 instead of 18? Wonder if they would refuse entries with the wrong answer?
Love it when people try to simplify and end up making things ambiguous. Why didn't they just do addition/subtraction?
The below, from about.com web site, expands on Patrick's previous answer and also addresses you question about the complexity of the math question. Interesting the games we play to fool ourselves...
"Canadian sweepstakes law, unlike American law, requires that the third component, "winners are chosen by luck," is removed. Sponsors cannot use pure luck to determine who wins a sweepstake. There must be at least some element of skill involved.
In order remove the element of pure chance, sponsors narrow the field of potential winners by requiring a skill testing question to enter their contests. Every entrant does not have the same chance to win; only those who at least pass the skill testing question are eligible to win prizes. Of course, this is only a technicality. Most people can pass the skill testing questions without difficulty, although sponsors are required to make the test somewhat challenging.
What Constitutes a Skill-Testing Question?
The courts have agreed that a four-part mathematical test such as "155 plus 33 divided by 2 minus 12" is enough to qualify as a skill-testing question.
An easy math testing question is the minimum required to hold a legal Canadian contest or sweepstakes. Some Canadian sweepstakes go a step farther and ask a trivia question or something a bit more difficult. Others are true contests, where the entrants are judged based on their skills."