You want the 20 degrees of temperature to play out over the 180 degrees of the servo.
You can use map()
servoAngle = map(temperature, 10, 30, 0, 180;
and you probably want to use constrain()
servoAngle = constrain(servoAngle, 0, 180);
because map() will cheerfully extrapolate and give servo angles out of range.
Oh, servopos not servoAngle, sry.
HTH
a7