Sally initially has 15 pencils. After giving 3 pencils to her friend, the number of pencils she has left can be calculated as follows:
15 pencils - 3 pencils = 12 pencils.
To find the fraction of pencils Sally has left in relation to the original number of pencils, we can express this as:
\[ \text{Fraction of pencils left} = \frac{\text{Number of pencils left}}{\text{Original number of pencils}} = \frac{12}{15}. \]
Now, we can simplify this fraction:
\[ \frac{12}{15} = \frac{4}{5} \quad \text{(by dividing both the numerator and denominator by 3)}. \]
Thus, the fraction of pencils that Sally has left is \(\frac{4}{5}\).