Answer: 260.869 bead
In this case, Kelly will use the same number of each type bead. Then you must calculate the bead mean length. The calculation will be:
2 bead= 4mm + 7.5mm
2 bead= 11.5mm
1 bead= 5.75m
Then, divide the bracelet length with bead mean length
15cm x 100mm/cm / (5.75/bead)= 260.869 bead.