15. \( (15)\left(\frac{2}{3}+\frac{1}{3}\right) \)

Let's break it down! When you see \( \frac{2}{3}+\frac{1}{3} \), you can easily add those fractions together since they share the same denominator. So \( \frac{2}{3}+\frac{1}{3} = \frac{3}{3} = 1 \). Therefore, the expression simplifies to \( (15)(1) \), resulting in \( 15 \). Math magic at work! Now, if you're ever in doubt about performing operations involving fractions, a nifty tip is to always find a common denominator first. This way, you can avoid errors and make the addition or subtraction of fractions smoother than a jazz piano solo! Just remember, practice makes perfect!