6. A necklace has a total of 99 beads, as shown in the figure, where the 1st bead is white, the 2nd and 3rd beads are red, the 4th bead is white, the 5th, 6th, 7th, and 8th beads are red, the 9th bead is white, ‥ then the necklace has a total of $\qquad$ red beads.
【Answer】Solution: The number of red beads increases in the pattern of $2, 4, 6, 8, 10$; their sum is to be found within 100.

If there are 9 groups of red beads, their sum is:
$$
2+4+\cdots+16+18=90 \text { (beads); }
$$

Adding 9 white beads in between, it makes exactly 99 beads; so there are 90 red beads.
The answer is: 90 .