I have a data set and am making vertical profiles (depth on y axis)

I am averaging freshwater fractions for different water layers. Instead of doing a straight average, I need a weighted one in relation to the depth intervals.

For example:

Depth fAltantic

0 1

25 2

50 3

75 4

125 6

If I take a straight average it is 3.2. But it should be 3.5 (assume a linear relationship and add a point of (100,5) My data is not this "perfect" what equation can I apply to obtain averages over depths?