Final answer:
Hassan should cut Sandy's hair to approximately 15 inches on the sides, accounting for shrinkage after drying, by cutting it slightly longer when wet and then trimming it to the desired length after it is dry.
Step-by-step explanation:
The scenario described does not provide clear details on the exact length of Sandy's hair prior to cutting. Hassan knows from their consultation that Sandy's desired finished hairstyle should be approximately 15 inches long on the sides. If Sandy's hair is currently longer than this desired length, Hassan will need to cut it down to achieve the 15-inch length after accounting for any potential shrinkage due to drying. It's important to allow for a small margin when cutting wet hair, as hair can appear longer when it's wet and may shrink a bit as it dries.
However, if Sandy's hair is already at the desired length when wet, Hassan might not need to cut it further until he can assess the true length once it's dry. In such cases, hairdressers typically dry the hair to see its natural fall and final length before finalizing the cut. Given the context of the question, Hassan should proceed with caution and potentially cut the hair slightly longer than 15 inches if it's currently wet, and then trim it to the desired length after drying if needed.