Answer:
After 15 minutes, the temperature of both the furnaces will be same.
Step-by-step explanation:
Let
be the minutes when the temperature of both becomes same.
Now, as per question:
Temperature of the furnace after
minutes that is cooling at the rate of 15 degree per minute is given as:

Temperature of the furnace after
minutes that is heating at the rate of 5 degree per minute is given as:

Now, equating both the temperatures, we get

Therefore, after 15 minutes, the temperature of both the furnaces will be same.