How to linear interpolate between two graph??

[saranyatl]

Dear All


I have plotted X against Y at different values of Z.

X	Y1 (Y @ Z=1)	Y2 (Y @ Z=2)	Y3 (Y @ Z=3)
20	2	4	6
40	4	6	8
60	6	8	10
80	8	10	12

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Now, I have to interpolate Y for any value between X=20 & X=80 and any value between Z1 & Z3.

say..What will be the Y value if X=35 and Z=1.5??