# Blog

We have a simple list of 4-digit alphanumeric codes like A001, A002, A003, etc. What formula can we use to mark codes are "out of sequence" with previous entries? This problem includes two separate challenges.

In this formula challenge, we have data showing cups of coffee sold at a small kiosk for a week. What formula will look up and sum total cups sold after 12:00 PM on Tuesday and Thursday? Relevant cells are shaded in green.

In this formula challenge, we have four tests (Test A, Test B, Test C, and Test D), all with the same 19 questions, but listed in a different order. What formula can be used to match questions and construct an answer key for all tests? A perfect use for Excel's powerful lookup formulas.

The challenge - calculate the difference from the current and last entry, even when previous entries are skipped. This is one of those problems that seems simple at first glance, but is tricky when you look closely.