Jenine can sharpen a pencil 5 times before it runs out. She needs to sharpen a pencil for every 1.5 hours of use. She already has ten pencils and needs to write for 105 hours. A new pencil costs $2. How much does she need to spend on more pencils to be able to write for 105 hours? Let's be accurate as possible.
A pencil last 7.5 hours because 5 x 1.5 = 7.5. The pencils she has will last 75 hours because 10 x 7.5 = 75. She needs 30 more hours of writing because 105 - 75 = 30. She will need 4 more pencils to reach this because 30 / 7.5 = 4. She will spend $8 on pencils because 4 x 2 = 8.
The answer: 8.