She planted eight trees. This means she used half of that amount of apples, which is 8 trees / 2 trees / apple = 4 apples. That means that she planted four of the six apples she bought, leaving only 6 apples - 4 apples = 2 apples to be eaten.
The answer: 2
Reverse engineering the question: Mary bought six apples from the store. From the apples she bought, for each that Mary ate, she planted two trees from the remaining ones. How many apples did Mary eat?

There are 21 * 2 = 42 polar bears. So the total 21 + 42 = 63 total animals.
The answer: 63
Reverse engineering the question: A zoo has 21 penguins and twice as many polar bears? How many total animals do they have?

If the class has 300 students, there are 40 / 100 * 300 = 120 smoking students. In a year, 70% of the smoking students are hospitalized from smoking - related complications, a total of 70 / 100 * 120 = 84 students. The number of smoking students who've not been hospitalized from smoking - related complications is 120 - 84 = 36.
The answer: 36
Reverse engineering the question: In a class of 300 students, the number of smoking teens is 40% of the class. In a year, 70% of the smoking students are hospitalized from smoking-related complications. Calculate the total number of smoking students from the class who have not been hospitalized from smoking-related complications.

Davante has 14 friends because 2 times 7 equals 14. Davante has 11 friends who are boys because 14 - 3 equals 11.
The answer: 11
Reverse engineering the question:
Davante has twice as many friends as there are days in the week. 3 of his friends are girls. How many friends does he have that are boys?