You would need about 34 2/3 tablespoons of butter. Convert the fraction into an improper fraction, so 8/3. Then multiply that by 13 (you and your brother are included in the 15 people I suppose) and you get 104/3. Simplify that to get a mixed number and you have 34 2/3.
If you use 2 2/3 tablespoons for you and your brother, that means, you need 2 2/3 tablespoons for two servings, how many tablespoons for 15 servings then?
For this case we have: 1 quarter: (1) * (0.25) = 0.25 $ 2 dimes: (2) * (0.1) = 0.2 $ 13 pennies: (13) * (0.01) = 0.13 $ Adding we have: 0.25 + 0.2 + 0.13 = 0.58 $ As a fraction: 58/10 = 29/5 $ Answer: The total amount of money as a fraction and as a decimal are: $ 29/5 $ 0.58