According to Newton’s law of cooling, the rate of loss of heat ∝cooling difference in temperature.
Here, average of 80°C and 50°C = 65°C
Temperature of surroundings = 20°C
∴ Difference = 65 – 20 = 45°C
Under these conditions, the body cools 30°C in 5 minutes.
∴

Change in temp.

Time

=kΔT
or

30

5

=k×45°C...(i)
The average of 60°C and 30° is 45°C which is 25°C (45 – 20) above the room temperature and the body cools by 30°C (60 – 30) in a time t (say).
∴

30

t

=k×25°C...(ii)
where k is same for this situation as for the original.
Divide (i) by (ii)

30∕5

30∕t

=

k×45

k×25

or

t

5

=

9

5

t=9‌min