+1557 The six faces of a cube are painted black. The cube is then cut into $3^3$ smaller cubes, all the same size.
(a) How many of the smaller cubes have exactly one black face?
(b) How many of the smaller cubes do not have any black faces?
(c) One of the small cubes is chosen at random, and rolled. What is the probability that when it lands, the face on the top is black?
+37147 If you start with a 3 x 3 x 3 cube , when you cut it up there will be
ONE cube on each face with only one painted side = six cubes with one painted side
Only the very center cube will have no painted sides
EIGHT cubes will have THREE painted sides
TWELVE cubes will have TWO painted sides
there are 27 cubes, randomly drawing a cube:
of the cubes you have a 6/27 chance of picking a 1 painted side prob of rolling paint side up = 1/6
1 / 27 zero painted side = 0/6
12/27 two painted sides = 2/6
8/27 three painted sides = 3/6
The the total probability of a painted side up is
6/27 * 1/6 + 1/27 * 0/6 + 12/27 * 2/6 + 8/27 * 3/6 = 1/3 chance of painted side up
Maybe a bit simpler : there are a total of 54 painted sides out of 162 smaller-cube sides black = 54/162 = 1/3
