Math problem?

Question

http://i2.photobucket.com/albums/y2/chontia/table.jpg
I have this table of data and I need to find the answers to the last column. How do I do this? Here's the question.
Using I(d) to represent the light intensity at the current depth of water, and I(d+1) to represent the light intensity at the next layer, calculate the difference between layers of plexiglas I(d+1) - I(d).

Answer 1

Think of the levels as numbered 1 through 10. Subtract the next level from the current level.