Newtons Law of Cooling is 
Here
is the temperature of the object and
is the surrondings temperature.
Change in temperature
.
At the intital time 
and
.
Substitute
and
in the equation
.
.
The general equation is
.
Substitute
in the above equation.

After three minutes, the temperature is
then
.
Hence
.
substitute
and
in the above equation
.
Substitute
in the equation
at
.


The equation is 
Find the temperature after
minutes.
The equation is
.
.
Temperature after
minutes :

Substitute
and
in the equation
.
.

Temperature after
minutes is
.
The equation is 
Find the temperature after
minutes.
The equation is
.
.
Temperature after
minutes :

Substitute
and
in the equation
.
.


Temperature after
minutes is
.
Temperature after
minutes is
and Temperature after
minutes is
.