[Question]Yuan is 7 years older than his brother David and twice his age. How old is David?
[Answer]Let x be David's age. Yuan is seven years older than David, 7 + x = Y years. Yuan is also twice David's age, 2 * x = Y years. Since these two equations equal the same thing, we can write: 7 + x = 2 * x. Subtracting x from both sides we get 7 = x, so David is 7 years old. The answer is 7.

[Question]Ever since she was a little girl, Sheila has dreamed of traveling the world. To help fund her dream, she bought a large piggy bank in December and started saving. By last week, she had saved $3,000. Pleased with her progress, she has decided to continue saving $276 per month, for 4 years. Today, Sheila’s family secretly added $7,000 into the piggy bank. At the end of 4 years, how much money will be in Sheila’s piggy bank?
[Answer]Today, Sheila’s piggy bank has $3000 + $7000 = $10000. Sheila will be saving for 4 years which is equivalent to 12 months / year * 4 years = 48 months. In 4 years, she will save $276 / month * 48 months = $13248. The piggy bank will have a total of $10,000 + $13,248 = $23248. The answer is 23248.

[Question]John buys a gaming PC for $1200. He decides to replace the video card in it. He sells the old card for $300 and buys a new one for $500. How much money did he spend on his computer, counting the savings from selling the old card?
[Answer]He spent an extra 500 - 300 = $200 on the video card. That means the total cost was 1200 + 200 = $1400. The answer is 1400.

[Question]Jonas is a book collector. He has so many books he is converting his third bedroom into a library. This room has 400 square feet of space. He plans to put several bookshelves in the room and each bookshelf takes up 80 square feet of space. If he reserves 160 square feet of space to use for a desk and some walking space, how many shelves can he put in the room?
[Answer]
If he reserves 160 sq. ft. of space for a desk and some walking space, then he will have 400 - 160 = 240 sq. ft. of space remaining. If each bookshelf takes up 80 sq. ft. of space, then 240 sq ft can contain 240 / 80 = 3 bookshelves. The answer is 3.