The distance between -10 and 110 degrees Celsius is 25 cm.?
The volumetric expansion of mercury in a thermometer is linear which is why the scale along the capillary into which the pool of mercury expands has equidistant spaces for a given temperature change. You can therefore equate the temperature range of 120 C into equidistant units of separation for each unit of temperature rise. Therefore a 15 cm distance from -10 C gives a temperature of: 15/25 x 120 C + (-10) C = 62 C The temperature range of the thermometer is already indicated in your question, namely -10 C to +110 C.