Where the maximum value is not 3 (only 1 or No.2 ball) and 2*2*2=8 kinds.
Subtraction gives 27-8= 19.
Count, No.3 ball taking method of 65438 +0 times +2 times +3 times =
C(3, 1)*C(2, 1)*C(2, 1)+C(3,2)*C(2, 1)+C(3,3)= 12+6+ 1= 19
That's right. I am also a senior three. It's not easy, dear!
I hope to adopt-