First subtract the total price ($160) by the price of the shirt ($20).
160 - 20 = 140
Now were left with $140. Since Katie bought 4 sweaters, divide 140 by 4.
140/4 = 35
This means that each sweater was $35. If you want to make sure this is correct just multiply 35 by 4 and then add the $20. You should end up with $160.
