o find the price of 15 pounds of grapes, you need to multiply $6 by the scale factor,

To find the price of 15 pounds of grapes, you need to multiply the price per 4 pounds by the scale factor \(\frac{15}{4}\).

The equation you're working with is:

\[ 6 \cdot \frac{15}{4} \]

To express this as a single fraction, you can multiply the whole number \(6\) by the fraction \(\frac{15}{4}\).

First, rewrite \(6\) as a fraction:

\[ 6 = \frac{6}{1} \]

Now, multiply the two fractions:

\[ \frac{6}{1} \cdot \frac{15}{4} = \frac{6 \cdot 15}{1 \cdot 4} = \frac{90}{4} \]

Now, simplify the fraction \(\frac{90}{4}\):

\[ \frac{90}{4} = \frac{45}{2} = 22.5 \]

Therefore, the price of 15 pounds of grapes is \(\frac{90}{4}\), or \(22.5\).

So, the complete equation can also be written as:

\[ 6 \cdot \frac{15}{4} = \frac{90}{4} \]

And the final answer is:

\[ \frac{90}{4} \text{ or } 22.5 \]