2 boxes, 1st contains 4 white & 6 black balls
2nd box contains 8 white & 8 black balls
We randomly draw a ball, got a black one
What′s probability that it was drawn from Box_2 ?
20 students should stand in 5
different rows. each row should have
at least 2 students. in how many ways
can you arrange them?
(an unsolved old question)