YES x(AB) == (xA)B for scalar x.

Recall that xA means that every element of A is multiplied by x. Here is a demonstration of the rule:

Often a matrix-matrix multiplication can be simpified by factoring one of the matrices. See the question.

Compute the following:

10 0 20 20 40 10 0 20 10 -2 2 1 1 2 -2

10 0 20 20 40 10 0 20 10

-2 2 1 1 2 -2