The data layout does not lend itself to a simple formula. Assuming that
the table is in A1:E5, you could interpolate with either

=FORECAST(5.5,B2:C2,B$1:C$1)+(2.5-A2)/(A3-A2)*(FORECAST(5.5,B3:C3,B$1:C$1)-FORECAST(5.5,B2:C2,B$1:C$1))
or
=(A3-2.5)/(A3-A2)*FORECAST(5.5,B2:C2,B$1:C$1)+(2.5-A2)/(A3-A2)*FORECAST(5.5,B3:C3,B$1:C$1)

In general you can use FORECAST for 1-D linear interpolation and TREND
for multi-D linear interpolation if the data are layed out appropriately.

Jerry

Geeps wrote:

I am interpolating in an array
for ex,
5 6 8 9
2 22.5 23.4 25.6 28
3 20.2 21.5 22.6 23
4 19 20 21.2 22
5 18 19.2 20 20.5

The First Row is (Xvalues) and the first coumn (Yvalues)
For ex, For X=5.5 and Y=2.5, the interpolated value will be 21.90. How can i
write a formula for this? i would appreciate your help.
Thanks so much in advance