A dozen = 12.
Two dozen = 24
Set up a proportion:
[tex] \frac{3 pens}{.99 cents} = \frac{24 pens}{X} [/tex]
Then we need to solve for X (the amount of money she paid)
Cross multiply to get 3x = 24(.99)
3x = 23.76
x= $7.92
Or you can do it another way if you knew 3 goes into 24 8 times, so you can multiply .99x8 = $7.92
