Answer: 10 inches
Explanation:
We know that each single Denalia shell is 2 inches. So let's calculate how many inches a 25 shell necklace would be
25(number of shells)×2(inches per 1 shell) = 50in
Next let's find the second necklace with 20 shells
20(number of shells) × 2(inches per 1 shell) = 40in
Finally we take the difference between the two
50in - 40in = 10 inches
Thus, the difference between a 25 shell necklace and a 20 shell necklace is 10 inches.