The total cost of maths books is 4 * 20 = $80$. She bought six more science books than maths books which totals 6 + 4 = 10 books. If each science book cost her $10 she spent 10 * 10 = $100 on science books. There were twice as many art books as maths books which total 2 * 4 = 8. The total cost for art books is 8 * 20 = $160. The total amount that she used for maths, science, and art books is 160 + 100 + 80 = $340. The amount she spent on music books is 500 - 340 = $160.
The answer: 160
Reverse engineering the question: June has $500 for buying school supplies for the new school year. She buys four maths books at $20 each, six more science books than maths books at $10 each, and twice as many art books as maths books at $20 each. If she also bought music books, how much money did she spend on music books?

The price of the tea was twice the price of the cheese, so the cheese cost was 10 / 2 = $5. The price of the butter was 80 / 100 * 5 = $4. If the bread was 2 times cheaper than butter, that means it cost 4 / 2 = $2. That means Ursula paid for all her shopping 2 + 4 + 5 + 10 = $21.
The answer: 21
Reverse engineering the question: Ursula went to the store and bought butter, bread, a package of cheese, and tea. The bread was 2 times cheaper than the butter, while the price of the butter was 80% of the price of cheese. Tea was the most expensive and cost twice the price of a package of cheese. If the tea cost $10, how much did Ursula pay for her purchases?

Lucca has 100 basketballs x (10 / 100) = 10 basketballs. Lucien has 200 basketballs x (20 / 100) = 40 basketballs. Together they have 10 basketballs + 40 basketballs = 50 basketballs.
The answer: 50
Reverse engineering the question: Lucca has 100 balls and 10 percent of his balls are basketballs. Lucien has 200 balls and 20 percent of them are basketballs. In total how many basketballs do Lucca and Lucien have?

Each day it needs to consume 7 / 7 = 1 gallon of blood. There are 128 / 16 = 8 pints per gallon. So he needs to suck blood from 8 / 2 = 4 people per day.
The answer: 4
Reverse engineering the question:
A vampire needs to suck 7 gallons of blood per week to survive. If he sucks 2 pints of blood per person, how many people does he need to suck each day to survive?