3. Ben has exactly the right number of cubes, each of side $5 \mathrm{~cm}$, to make a solid cube of side $1 \mathrm{~m}$. He places the smaller cubes side by side to form a single row. How long is this row?
A $5 \mathrm{~km}$
B $400 \mathrm{~m}$
C $300 \mathrm{~m}$
D $20 \mathrm{~m}$
E $1 \mathrm{~m}$
3. B The number of small cubes along each edge of the large cube is $100 \div 5=20$. Therefore Ben has $20 \times 20 \times 20=8000$ small cubes in total. Hence the row he forms is $8000 \times 5 \mathrm{~cm}=40000 \mathrm{~cm}$ long. Since there are $100 \mathrm{~cm}$ in $1 \mathrm{~m}$, his row is $400 \mathrm{~m}$ long.