I usually use geometric block to do multiplication. $4\times 5$ is the number of $1\times 1$ blocks inside a rectangle with sides $4$ and $5$ that is $20$ $1\times 1$ blocks. in the case of $0.5\times 0.5$ we have a square with side $0.5$ and we want to know the number of $1\times 1$ blocks inside that.
