Write a proportion that can be used to find the cost of 10 notebooks if 3 notebooks cost $1.98.
Let x represent the cost of 10 notebooks.
3 notebooks / $1.98 = 10 notebooks / x
Cross multiply: 3x = $1.98 * 10
3x = $19.80
x = $6.60
Therefore, the cost of 10 notebooks is $6.60.