A glass flask has a volume of 900 ml at a temperature of 20° C. The flask contains 895 ml of mercury at an equilibrium temperature of 20°C. The temperature is raised until the mercury reaches the 900 ml reference mark. The coefficients of volume expansion of mercury and glass are 18 x 10-5 K-1 and 2.0 x 10-5 K-1, respectively. The temperature at which this occurs, in °C, is closest to: