Trying find out which is true

How do you use the elimination method for this question? explain, because i really want to understand!

1. writing an equation for an exponential function by 2. a piece of paper that is 0.6 millimeter thick is folded. write an equation for the thickness t of the paper in millimeters as a function of the number n of folds. the equation is t(n)= 3. enter an equation for the function that includes the points. (-2, 2/5) and (-1,2)

In a class, there are 12 girls. if the ratio of boys to girls is 5: 4 how many boys are there in the class