The pair is not in sequential order, you would have to find them yourself, as written below...

Trouble is that he mixed all the prices (initial and discounted), so now he needs your cool coding skills to filter out only the discounted prices.

Where do you see that if the two names have the same length you should return an empty array? Read the description again, if the names have a length of 4, then they are your friends and they should be in the output array.