[Question]: Eustace is twice as old as Milford. In 3 years, he will be 39. How old will Milford be?
[Answer]: Eustace’s current age must be 39 years old – 3 years = 36 years old. So Milford’s current age must be 36 years old / 2 = 18 years old. So in 3 years, Milford will be 18 years old + 3 years = 21 years old.
So the answer is 21.

[Question]: In a big box, there are marbles in 4 colors. Inside, there are 20 red marbles. There are three times more green marbles than red. Yellow marbles make up 20% of the green marbles. How many marbles are of a different color if the total of all marbles in the box is three times more than the number of green marbles?
[Answer]: There are 3 * 20 = 60 green marbles. There are 60 * 20 / 100 = 12 yellow marbles. In total there are 3 * 60 = 180 marbles. That leaves 180 - 20 - 60 - 12 = 88 marbles of another color in the box.
So the answer is 88.

[Question]: Arthur has $200 in his wallet. He spends four-fifths of that. How much does he have left?
[Answer]:
Arthur spent 4 / 5 * $200 = $160. Thus he has $200 - $160 = $40 remaining.
So the answer is 40.