Fineness model 2.8-3.1 sand rate 34-38 that fineness model 2.9 use interpolation method to calculate sand rate? The formula is also written out!

Fineness model 2.8-3.1 sand rate 34-38 that fineness model 2.9 use interpolation method to calculate sand rate? The formula is also written out!

x1 = 2.8
y1 = 34
x2 = 3.1
y2 = 39
When x = 2.9
y = (x - x1)(y2 - y1) / (x2 - x1) + y1
= (2.9 - 2.8)(39 - 34) / (3.1 - 2.8) + 34
= 35.667
Fineness model 2.9 the sand rate calculated by interpolation method is 35.667