I am taking a mathmatics course at my local college and I have a problem that I can not figure out.?

The question is:





Mercury, Venus, and Earth revolve around the sun once every 3,7 and 12 months, respectively. If the three planets are now in the same straight line, what is the smallest number of months that must pass before they line up again?





Can some body please help!!! It's my last one and I'm finished.I am taking a mathmatics course at my local college and I have a problem that I can not figure out.?
You need the smallest number for which 3, 7, and 12 are divisors. This ';least common multiple'; is 3 * 7 * 4 = 84 months.I am taking a mathmatics course at my local college and I have a problem that I can not figure out.?
Try Greatest Common Factor
3, because they pass every 3, 7, and 12 months. The smallest number they pass is every 3 months, sooo....





Hope I could help! By the way, what kind of math is this, Calculus? :)?