Question 15875
Let Number of pencils bought be 'x' and pens be 'y'.
Then 25x + 80y = 2000 ----- (1) and x + y = 36 ------- (2)
5x + 16y = 400 [Divide eq (1) with 5.]
5x + 5y  = 180 [Multiply eq (2) with 5.]

     11y = 220
       y = 20
 x + 20 = 36  [Use eq(1).]
       x = 16
Therefore, Susan bought the number of pencils are 16.

And

Let the two numbers be 8x  and 5x.
8x - 5x = 24
     3x = 24
      x = 8
Therefore, the numbers are 64 and 40.