How can i explain how you can add two fractions with denominators of 10 and end up with a sum whose denominator is 5 me

How is working backward similar to solving an equation?

Sanya noticed that the temperature was falling at a steady rate of 1.4 degrees every hour from the time that she first checked her outdoor thermometer. by 6 a.m., the temperature had fallen 21 degrees. which expression can you use to find how many hours earlier she had first checked the thermometer?

Ineed to know the answer to this algebra question.