YES c(AB) =  (cA)B for scalar c.
Recall that cA means that every element of A is multiplied by c. 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