Interpolating Data In Excel Table With Missing Data

I have a table with 21 numbers two rows with x and y e.g.

nr.xy
205,5
195,4
185,3

so on ........

Now I wish to make a table with 100 numbers in wish y is interpolated based on y in the given table. Using many IF statements (in Dutch "ALS") isn't useful since the number of statements is becoming too large and besides is a number is missing e.g. nr7 x=0,7 the y is 0.

This is not working properly:
=
ALS(P12<$H$28;"Bodem";
ALS(P12<$H$27;$i$28;
ALS(P12<$H$26;$i$27;
ALS(P12<$H$25;$i$26;
ALS(P12<$H$24;$i$25;
ALS(P12<$H$23;$i$24;

[Code] .....