Colin is to build a large Cube using smaller cubes. How many cubes one foot on a side will Calin need to make a cube one yard on a side? a. 21 cubes
b. 25 cubes.
c. 27 cubes
d. 30 cubes​

Since 3 feet = 1 yard, Colin needs 3 cubes on each edge.  (See picture below.)

In that picture, you'll also see that there are 9 cubes on one layer and there are three layers.  

9x3 = 27

use this formula:

p(a\b) = p(a and b)/p(b).

take it from here.