The number of birthday presents that David found is 60 / 2 = 30. For his birthday and Christmas, David found a total of 30 + 60 = 90 presents.
The answer: 90
Reverse engineering the question: The number of Christmas presents David found was twice as many as the number of birthday presents. If David found a total of 60 Christmas presents, how many presents did he get all together?

For every addition of 100 people added the speed is halved so for the first 100 people added its speed is 1 / 2 * 500 km / hr = 250 km / hr. For another addition of 100 people its speed is also halved, 1 / 2 * 250 km / hr = 125 km / hr. So for the total of 200 additional people, its speed is 125 km / hr.
The answer: 125
Reverse engineering the question: For every 100 additional people that board a spaceship, its speed is halved. If the speed of the spaceship with 200 people on board is 500km per hour, what is its speed in km/hr when there are 400 people on board?

The first has an area of 20 square inches because (4 x 5) = 20. The second has an area of 18 square inches because (3 x 6) = 18. The difference in area is 20 - 18 = 2 square inches.
The answer: 2
Reverse engineering the question: Toby has two rectangles of cloth. The first is 4 inches wide and 5 inches tall. The second is 3 inches wide and 6 inches tall. How many more square inches is the area of the first compared to the second?

First find the total number of starfish arms: 7 starfish * 5 arms / starfish = 35 arms. Then add the number of seastar arms to find the total number of arms: 35 arms + 14 arms = 49 arms.
The answer: 49
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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Carly collected 7 starfish with 5 arms each and one seastar with 14 arms. How many arms do the animals she collected have in total?