When two fractions are between 0 and 1/2, how do you know which fraction is greater? explain

Carlos has 4/5 of a tank of fuel in his car. he uses 1/10 of a tank per day. how many days will his fuel last?

An advertising firm has an old computer that can prepare a whole mailing in 6 hours. with the of newer model the job is complete in 2 hours. how long would it take the newer model to do the job alone?

Offering 30 if a plus b plus c equals 68 and ab plus bc plus ca equals 1121, where a, b, and c are all prime numbers, find the value of abc. the answer is 1978 but i need an explanation on how to get that.