Imagine a metal rod 10.000m long at 20.000c. Suppose that this expands to a length of 10.025 meters at 25.00oc. What is the thermal coeficienty of linear expansion?

Divide the change in length by the original length to get % elongation, then divide by the change in degrees C to get % elongation per degree C. That's the coefficient of thermal expansion that you're trying to calculate.