We can start by making an equation from the information given on the month of March to find how many packs he bought.
[tex]4(1.35)+x(1.05)=8.55[/tex] This equation shows that for the first 4 packs he bought, he payed $1.35. Then, consequently, for each pack after that, he payed $0.30 less, and we don't know how many more he bought, so we set it up as shown. Finally, we set it all equal to $8.55 since that's how much he payed that month. We get that x=3, meaning that he bought a total of 7 packs of pens that month.
Knowing this, we can now make an equation for the month of September.
[tex]4(1.50)+3(1.20)=x[/tex] This time, we know the entire left side, that he paid $1.50 for the first 4 packs and $0.30 less for the last 3. We get x=$9.60
Knowing this, we can subtract the amount he paid in September by the amount he paid in March to get the answer... $9.60-$8.55=$1.05
I hope this helped!
