1. The problem states Erin has 55 pebbles in total. 2. She takes out her favorite 7 pebbles, so the remaining pebbles to divide are $$55 - 7 = 48$$. 3. These 48 pebbles are divided equally among her 3 sisters, so each sister gets $$\frac{48}{3} = 16$$ pebbles. 4. Su, the youngest sister, already had 15 pebbles. 5. After receiving her share, Su's total pebbles are $$15 + 16 = 31$$. **Final answer:** Su now has 31 pebbles.