Betty is saving money for a new wallet which costs $100. Betty has only half of the money she needs. Her parents decided to give her $15 for that purpose, and her grandparents twice as much as her parents. How much more money does Betty need to buy the wallet? Let's be accurate as possible.
In the beginning, Betty has only 100 / 2 = $50. Betty's grandparents gave her 15 * 2 = $30. This means, Betty needs 100 - 50 - 30 - 15 = $5 more.
The answer: 5.