Simone = (1 / 2) * 16 = 8. Lauri = (1 / 3) * 15 = 5. 8 + 5 = 13. Together they ate 13 apples.
The answer: 13
Reverse engineering the question: Simone ate 1/2 of an apple each day for 16 days. Lauri ate 1/3 of an apple each day for 15 days. How many apples did the two girls eat altogether?

Twice Angie's age is 20 - 4 = 16. Angie is 16 / 2 = 8.
The answer: 8
Reverse engineering the question: Twice Angie's age, plus 4, is 20. How old is Angie?

2 / 3 students are girls because 1 - (1 / 3) = 2 / 3. 20 students are girls because 30 x (2 / 3) = 20. 40% of the girls don't have any pets because 100 - 40 - 20 = 40. 8 girls in class have no pets because 20 x .4 = 8.
The answer: 8
Reverse engineering the question: In a classroom there are 30 students. 1/3 of them are boys. Among the girls, 40% own dogs, 20% own a cat, and the rest have no pets. How many girls in the class have no pet?

He had 80 - 20 = 60 after the donation. He kept 60 / 2 = 30 after giving half to his brother. So he has 30 * 3 = 90 coins. He then does 1 more donation and has 90 - 10 = 80 coins left.
The answer: 80
Reverse engineering the question:
Jake amasses a fortune of 80 bitcoin. He donates 20 bitcoins to charity. He then gives half of all the bitcoins to his brother. After that, he triples the number of bitcoins he has. Then he donates another 10 coins. How many coins does he have?