When you use a transversal to construct a line parallel to a given line, how do you prove that the lines are parallel?

Parallel lines never touch each other.

Principal amount invested = 2500
Rate of interest given = 8%
Time for which the money is kept = 15 years
Then
Interest = Principal * Rate * Time
             = 2500 * 4% * 15
             = (2500 * 4 * 15)/100
             = (25 * 4* 15)
             = 1500
So the interest gained by me is 1500. I hope this is the answer you were looking for and the procedure for doing the problem is clear to you. In future you can do such problems without requiring any help.