A good equation would be: f(<em>x</em>)=<em>P</em>(<em>r</em>)^(15). The full equation would be f(<em>x</em>)=<em>P</em>(<em>r</em>/1)^(1(15)), but I just simplified it. <em>P</em> is the initial/principal population of the snails, and <em>r</em> is the yearly rate of growth.
15/2 r -40, the first answer
Factor them
45=3*3*5
18=2*3*3
lcf=2*3*3*5=90