To find the cost of 10 pencils, we can set up a proportion based on the information given. We know that 3 pencils cost $1.50, and we want to find out how much 10 pencils will cost (let's call that amount \( x \)).
The proportion can be set up as follows:
\[ \frac{3 \text{ pencils}}{1.50 \text{ dollars}} = \frac{10 \text{ pencils}}{x \text{ dollars}} \]
Now we can cross multiply to solve for \( x \):
\[ 3 \cdot x = 10 \cdot 1.50 \]
Calculating the right side:
\[ 10 \cdot 1.50 = 15 \]
So now we have:
\[ 3x = 15 \]
To solve for \( x \), divide both sides by 3:
\[ x = \frac{15}{3} = 5 \]
Thus, the cost of 10 pencils is $5.
Therefore, the answer is:
$5