61. There are 150 black and white chess pieces in total, divided into 50 piles, with 3 pieces in each pile. Among them, there are 15 piles with only 1 white piece, and 25 piles with no less than 2 white pieces. The number of piles with only white pieces is twice the number of piles with only black pieces. In these 150 chess pieces, the number of black pieces is $\qquad$.
Answer: 65