The question seems incomplete, but if you are trying to figure out how much each dress costs, The answer would be $30.
To find the answer all you need to do is; 1. Subtract the price of the jeans ($40) From the total price. 160-40=120
2. Now we know the total price of the Dresses, and we also know she bought 4 of them, this means we can divide the total price ($120) of dresses, by the number (4) of dresses bought. 120/4 = 30.