=percentile(A:A,1-percentrank(B:B,D2,30))

with data in columns A and B eg D2=65% gives 2.75
To find a y-value given an x-value, switch A:A and B:B

**Note that this formula assumes there is a decreasing relationship
and will return the same answer regardless of how data is ordered.
If there is an increasing relationship, remove the "1-".

"Soccer boy" wrote in message
...
i have a question. i am trying to find a function or some way to help me
interpolate a range of numbers to find a given value. say i have the
range
of numbers below.

X Y
1 100%
1.5 90%
2 80%
2.5 70%
3 60%
3.5 50%
4 40%
4.5 30%
5 20%

Now i want to find the value of X that corresponds to a Y value of 65%
now
obivioulsy the value is going to be 2.75, but say i have a harder range of
numbers is there a way i can do this.