8. Aladdin found 20 necklaces in the treasure room, some of which are set with 2 diamonds, and some are set with 5 diamonds, with a total of 79 diamonds. So, the number of necklaces set with 5 diamonds is $\qquad$ pieces
【Answer】 13
【Solution】 Consider the problem as a "chicken and rabbit in the same cage" problem. Assume all necklaces are set with 2 diamonds, the total number of diamonds would be: $20 \times 2=40$; the number of necklaces set with 5 diamonds is $(79-40) \div(5-2)=13$ pieces.