80 6 = 480 which is the total price for all of the 6 sculptures together if they were bought. You have to mulitply .85 to 480 because it represents how many weeks the price has gone down.
.480 .85 = 408
You then mulitiply 40 with .85 because that's new weeks price.
408 .85 = 346.8
You keep doing that until you get to, are are close to $270
346.8 .85 = 294.78
294.78 * .85 = 250.56
So your answer would be the 3rd week