Final answer:
The price at which the article was marked is $600.
Step-by-step explanation:
To find the price at which the article was marked, we need to reverse the discount calculation. The customer purchased the article for $510 after a 15% discount, so the marked price would be the original price before the discount.
To reverse the 15% discount, we need to divide $510 by 0.85 (100% - 15%). This gives us $600, which is the price at which the article was marked.