For this example I had 100 percent values in column A from cell A2 to cell A101.
I had steam pressure values in the corresponding cells in column B.
Next I created a table of percentage ranges:
(Note you probably don't want overlapping ranges - 0->10 and 10->30. i.e. 10 should not appear in both ranges, so either use 0->10 and 10.1->30 (or similar) or change the proposed formulas to use > instead of >= )
Table in cells D1 to I2:
D E F G H I
1 0 10 30 50 70 90
2 9.9 29.9 49.9 69.9 89.9 100
In cell D3 enter this formula:
Now drag the formula to extend it to cell I3.
You will have average pressure values for each range.
The first SUMPRODUCT totals the pressure values in column B for all rows in the specified percentage range in column A and the second SUMPRODUCT counts the total number of rows within the specified percentage range.