The 4 chairs cost Cory $135 - $55 = $80. Each chair costs $80 / 4 = $20.
The answer: 20
Reverse engineering the question: Cory bought a patio table and 4 chairs for $135. The patio table cost $55. If each chair cost the same amount, how much did each chair cost?

20 of 50 marbles are white so there are 50 - 20 = 30 red and blue marbles. The number of red marbles and blue marbles is the same, so there are 30 / 2 = 15 of each color. The difference between the number of white marbles and blue marbles is 20 - 15 = 5. Double the difference is 5 * 2 = 10 marbles. 10 marbles are removed to leave 50 - 10 = 40 marbles.
The answer: 40
Reverse engineering the question: There are 50 marbles in a box which has a mixture of white, red and blue marbles. There were originally 20 white marbles and an equal number of red and blue marbles. Jack removes a number of marbles equal to double the difference between the number of white marbles and the number of blue marbles. How many marbles are left in the box?

In the first store, each apple costs $3 / 6 = $0.5. In the second store, each apple costs $4 / 10 = $0.4. So, you can save $0.5 - $0.4 = $0.1 on each apple from buying $4 for 10 apples than from $3 for 6 apples. In cents, this is equal to $0.1 x 100 = 10.
The answer: 10
Reverse engineering the question: The first store charges $3 for 6 apples while the second store charges $4 for 10 apples. How many cents can you save on each apple from buying $4 for 10 apples than from $3 for 6 apples?

Since there are 7 months from January to July, then Roe's savings for those months sum up to $10 x 7 = $70. Since there are 4 months from August to November, then her savings for those months sum up to $15 x 4 = $60. Her total savings from January to November is $70 + $60 = $130. Therefore, she needs to save $150 - $130 = $20 in December.
The answer: 20
Reverse engineering the question:

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.
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Roe saved $10 per month from January to July and saved $15 per month from August to November. How much should she save in December so she will have a total savings of $150 in the year?