Anders,
my interpretation of the respective group is the forecast temperature for 1500H UTC is 18 °C and the forecast temperature for 1800H UTC is 16 °C.
ICAO Annex 3 states forecast temperatures, though not an element of a standard TAF, may be included in accordance with regional air navigation agreements. If they are given in a TAF it is recommended to indicate the maximum and minimum temperatures for the forecast period.
The format of the temperature group in your example, however, is not in line with ICAO Annex 3 nor the WMO Manual on Codes in their current versions. I take it the example is from a question bank and might be outdated.
If the temperatures in your example were indeed maximum and minimum temperatures at 1500H and 1800H this would have to be indicated today as "TX18/1500Z TN16/1800Z", with "TX" standing for maximum and "TN" standing for minimum temperature.