A body cools from 500C to 400C  in 5 min. The surrounding temperature is  200C. In what further time (in minute) will it cool to 300C?

Applying,   T1−T2t=αT1+T22−T0, we get 50−405=α50+402−20                   …(i)and   40−30t=α40+302−20             …(ii)Solving these two equations, we gett=253min