1. **Problem statement:** Alex threads white and black beads alternately onto a string. Two groups of 5 beads each are hidden (covered). We need to find how many white beads are hidden in total. 2. **Understanding the pattern:** The beads alternate colors: white, black, white, black, and so on. 3. **Key fact:** In any sequence of alternating beads, every two beads contain exactly one white bead and one black bead. 4. **Analyzing the hidden beads:** Each hidden group has 5 beads. Since 5 is an odd number, the number of white beads in each group will be either 3 or 2 depending on whether the first bead in the hidden group is white or black. 5. **Using the visible beads to determine the color of the first hidden bead:** The visible beads before the first hidden group end with black (since the pattern is white, black, white, black, then hidden). Therefore, the first hidden bead after a black bead must be white (because the pattern alternates). 6. **Counting white beads in each hidden group:** - For 5 beads starting with white, the sequence is: white, black, white, black, white. - This means 3 white beads per hidden group. 7. **Total white beads hidden:** There are 2 hidden groups, each with 3 white beads. $$\text{Total white beads hidden} = 2 \times 3 = 6$$ **Final answer:** 6 white beads are hidden in total.