A cuboid has length $4a$, breadth $3a$ and height $2b$. Write an expression for its volume.

Volume = length × breadth × height = $4a\times 3a\times 2b$. Coefficients: $4\times 3\times 2=24$. Variables: $a\times a\times b=a^{2}b$. So the volume is $24a^2$$b$. Writing $24ab$ forgets that $a\times a=a^2$; $9ab$ comes from adding 4 + 3 + 2; $a^3$ would need three a factors, but there are only two.