Answer: 105/512
Step-by-step explanation:
Multiply out the fractions. This is because volume = length*width*height when it comes to rectangular prisms, aka blocks.
(3/8)*(5/8)*(7/8) = (3*5*7)/(8*8*8) = 105/512
This fraction cannot be reduced since 105 and 512 have no factors in common, other than 1. A clue of this is to notice how neither 3, nor 5, nor 7 share any common factors (other than 1) with 8 in the denominator.