Describe your algorithm in words. How does it go from the values 0, 10 and 20 to outcome 10?

If you want to weigh them equally, you can just calculate the average.

If you don't want to weigh them equally, you need more input parameters for the weight than just one.

Ok, The last value is the frac value and it goes from 0 to 1.

Your 3-value examples are conveniently chosen. It's not clear if you are either not using the middle value or want interpolation between 2 chosen values, based on the (0-1) range of the weight.

What should be the result of these:

blend(0, 1, 20, 0.25) = ?
blend(0, 19, 20, 0.25) = ?

amnon.owed i want  interpolation between 2 chosen values, based on the (0-1) range of the weight.

In that case, you can use this method...

Code Example

1. void setup() {
2.   println( blendValues(0, 10, 20, 0.5) );  // 10
3.   println( blendValues(0, 10, 20, 0.25) ); // 5
4.   println( blendValues(0, 10, 20, 1.0) );  // 20
5.   println( blendValues(0, 1, 20, 0.25) );  // 0.5
6.   println( blendValues(0, 19, 20, 0.25) ); // 9.5
7.   exit();
8. }
9.  
10. float blendValues(float a, float b, float c, float t) {
11.   if (t<0.5) {
12.     return lerp(a, b, t/0.5);
13.   } else if (t>0.5) {
14.     return lerp(b, c, (t-0.5)/0.5);
15.   } else {
16.     return b;
17.   }
18. }