Let me help you with this question as well.
Given the total amount of money collected for the number of items sold, we can work out the exact amount collected for each type of item; shirts and dresses.
Knowing that there were also 3 times as many shirts sold, there were 3 units of shirts sold, and 1 unit of dresses sold. If we were to compare 1 unit of shirts, and 1 unit of dress, it would also be the same as comparing the same quantity for each type of item.
1 unit of shirts will be equivalent to $450 collected, since 3 units is $1350 collected.
1 unit of dresses will be equivalent to $630 collected in this case.
To compare 1 unit of shirts and dresses each ( that means same number of shirts and dresses!), we can see that there is $180 more collected from the dresses (630 – 450).
We know that for each dress, $12 more was collected, compared to each shirt. So, if $180 more was collected for each unit of shirts and dresses, if we divide $180 by $12, we know that the answer is 15. That means that there were 15 items per unit.
So, from here, we know that 15 dresses costs $630, 1 dress would be $42.
Hope this helps! Feel free to PM me (: