What equations are associated with the different types of variation? How do you know which to use?

The size of a certain cell is 2.5*10^-9m. another cell is 1.5*10^3 times larger. how large is the larger cell in scientific notation?

Acar travels at an average speed of 56 miles per hour. how long does it take to travel 196 miles

Suzanne began a trip with 18 1/2 gallons of gas in the gas tank of her car if she hughes 17 3/4 gallons on the trip how many gallons does she have left once she finish the trip