Calibration procedure:
The transfer function for this problem is of the form Y = bX + c where

the slope of the line (b) is (Y2-Y1)/(X2-X1) = b
                              (20-0)/(40-0)  = 0.5

and the Y-intercept (c) is ((Y2+Y1)/2) - b * (X2+X1)/2 = c
                                  20/2 - 0.5 * 40/2    = c
                                    10 - 10            = 0

therefore the transfer function is	Y = 0.5 * X + 0	
                                  12.5 oC = 0.5 * 25 + 0

and for 25% Globegerina the sea-surface temperature was 12.5 oC