A body at a temperature of `50^0F` is placed outdoors where the temperature is `100^0F` . If the rate of change of the temperature of a body is proportional to the temperature difference between the body and its surrounding medium. If after 5 min the temperature of the body is `60^0F` , find (a) how long it will take the body to reach a temperature of 75 `^0` F and (b) the temperature of the body after 20 min.