[Question]: If Beth is 18 years old and her little sister is 5, in how many years would she be twice her sister's age?
[Answer]: The age difference between Beth and her sister is 18 - 5 = 13 years. Beth must be 13 x 2 = 26 to be twice her sister's age. Beth will be twice her sister's age in 26 - 18 = 8 years.
So the answer is 8.

[Question]: John decides to buy utensils. They come in 30 packs with an equal number of knives, forks, and spoons. How many packs does he need to buy if he wants 50 spoons?
[Answer]: Each pack contains 30 / 3 = 10 spoons. So he needs to buy 50 / 10 = 5 packs.
So the answer is 5.

[Question]: The distance between Robin's house and the city center is 500 meters. He leaves the house to go to the city center. After he walks 200 meters he realizes that he forgot his bag. So he returns to his house, then goes back to the city center. How many meters does he walk in total?
[Answer]:

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
user
He walks 200 meters first then he returns to the house. So it makes 200 m + 200 m = 400 m. The city center is 500 meters away from his house so the total distance is 400 m + 500 m = 900 m.
So the answer is 900.