A cup of coffee cools from 90°C to 80°C in t minutes, when the room temperature is 20°C. The time taken by a similar cup of coffee to cool from 80°C to 60°C at a room temperature same at 20°C is : 

13t/5

From Newton's Law of cooling :

[\(\frac{T_1 + T_2}{2}\) - T_s ]K = \(\frac{T_2 - T_1}{\Delta t}\)

T₁ = 90 ; T₂ = 80 ; T_s = 20

K = -2/13t

Also, [\(\frac{80 - 60}{2}\) - Ts ]K = \(\frac{60 - 80}{t'}\)

t' = 13t/5