A battalion, 20 miles long, advances 20 miles. During this time, a messenger on a horse travels from the rear of the battalion to the front and immediately turns around, ending up precisely at the rear of the battalion upon the completion of the 20-mile journey. How far has the messenger traveled?

Differential equations: Newton's Law of Coolin

Need help on this..DE Newton's Law of Cooling

A thermometer is removed from a room whose ambient temperature is 70° F and is taken outside.
After one minute the thermometer reads 60° F and after three minutes, it reads 55° F. What is the outside
temperature (which is assumed to be constant)
? Find your answer by solving DE model of Newton’s Law
of cooling.