'the scale of a mercury-in-glass thermometer is linear. one such thermometer has a scale extending from -10 degrees Celsius to 110 degrees Celsius. the length of the scale is 240mm.'

what is linear? what is meant when the question said the scale is linear? how do i calculate the distance moved by the end of the mercury thread when temperature rises?

Linear means that the change in temperature is proportional to the distance that the column of mercury moves. The scale that is marked every two degrees (for example) has the same distance between each pair of marks. In your example, a 240 mm length of scale corresponds to a temperature change of 120 C, which corresponds to 2.0 mm per degree from -10 to 110 C.

so linear graphs means that the line on that graph is straight? and so do i use 240mm to divide by the change in degrees or the other way round to get the sensitivity of the thermometer?