The 12 is multiplied by 4 to get 48, and
the (5r + 6t) is multiplied by 6 to get (30r + 36t), therefore
you are multiplying the outside by 4 and the inside by 6, putting
those together is 24 for a factor.
If 12(5r + 6t) = w, then
48(30r + 36t) is equal to 24w.
