Three perfect cube blocks of different volumes were stacked on top of each other. the top block was 64 cubic centimeters, the middle block was 125 cubic centimeters, and the biggest block was 729 cubic centimeters. how tall was the stack of blocks?

We know that 

[volume of a cube]=b³> b=∛Volume
b> is the side length of a cube

The top block was 64 cm³> b1=∛64> b1=4 cm
The middle block was 125 cm³> b2=∛125> b2=5 cm
The biggest block was 729 cm³> b3=√729> b3=9 cm

[the stack of blocks tall]=b1+b2+b3> 4+5+9> 18 cm

the answer is
the stack of blocks was 18 cm tall

The answer would be $68.04