An air heated ventilation system will be installed in a workshop whose air change number is determined as 15 times / h and whose criteria is 15 x 18 x 4 m. The temperature is –10 ° C, the relative humidity is 50% outside air, and the indoor return air whose temperature is 25 ° C and the relative humidity is 70% is mixed and blown into space. Since it is known that the mixture's outdoor air ratio is 20%, what should the blowing temperature be?